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General Python FAQ

Date: 2005-03-07
Version: 1.23
Web site:http://www.python.org/

Contents

1   General Information

1.1   What is Python?

Python is an interpreted, interactive, object-oriented programming language. It incorporates modules, exceptions, dynamic typing, very high level dynamic data types, and classes. Python combines remarkable power with very clear syntax. It has interfaces to many system calls and libraries, as well as to various window systems, and is extensible in C or C++. It is also usable as an extension language for applications that need a programmable interface. Finally, Python is portable: it runs on many Unix variants, on the Mac, and on PCs under MS-DOS, Windows, Windows NT, and OS/2.

To find out more, start with the Beginner's Guide to Python.

1.2   Why was Python created in the first place?

Here's a very brief summary of what started it all, written by Guido van Rossum:

I had extensive experience with implementing an interpreted language in the ABC group at CWI, and from working with this group I had learned a lot about language design. This is the origin of many Python features, including the use of indentation for statement grouping and the inclusion of very-high-level data types (although the details are all different in Python).

I had a number of gripes about the ABC language, but also liked many of its features. It was impossible to extend the ABC language (or its implementation) to remedy my complaints -- in fact its lack of extensibility was one of its biggest problems. I had some experience with using Modula-2+ and talked with the designers of Modula-3 and read the Modula-3 report. Modula-3 is the origin of the syntax and semantics used for exceptions, and some other Python features.

I was working in the Amoeba distributed operating system group at CWI. We needed a better way to do system administration than by writing either C programs or Bourne shell scripts, since Amoeba had its own system call interface which wasn't easily accessible from the Bourne shell. My experience with error handling in Amoeba made me acutely aware of the importance of exceptions as a programming language feature.

It occurred to me that a scripting language with a syntax like ABC but with access to the Amoeba system calls would fill the need. I realized that it would be foolish to write an Amoeba-specific language, so I decided that I needed a language that was generally extensible.

During the 1989 Christmas holidays, I had a lot of time on my hand, so I decided to give it a try. During the next year, while still mostly working on it in my own time, Python was used in the Amoeba project with increasing success, and the feedback from colleagues made me add many early improvements.

In February 1991, after just over a year of development, I decided to post to USENET. The rest is in the Misc/HISTORY file.

1.3   What is Python good for?

Python is a high-level general-purpose programming language that can be applied to many different classes of problems.

The language comes with a large standard library that covers areas such as string processing (regular expressions, Unicode, calculating differences between files), Internet protocols (HTTP, FTP, SMTP, XML-RPC, POP, IMAP, CGI programming), software engineering (unit testing, logging, profiling, parsing Python code), and operating system interfaces (system calls, filesystems, TCP/IP sockets). Look at the table of contents for the Library Reference to get an idea of what's available. A wide variety of third-party extensions are also available. Consult the Python Package Index to find packages of interest to you.

1.4   How does the Python version numbering scheme work?

Python versions are numbered A.B.C or A.B. A is the major version number -- it is only incremented for really major changes in the language. B is the minor version number, incremented for less earth-shattering changes. C is the micro-level -- it is incremented for each bugfix release. See PEP 6 for more information about bugfix releases.

Not all releases are bugfix releases. In the run-up to a new major release, a series of development releases are made, denoted as alpha, beta, or release candidate. Alphas are early releases in which interfaces aren't yet finalized; it's not unexpected to see an interface change between two alpha releases. Betas are more stable, preserving existing interfaces but possibly adding new modules, and release candidates are frozen, making no changes except as needed to fix critical bugs.

Alpha, beta and release candidate versions have an additional suffix. The suffix for an alpha version is "aN" for some small number N, the suffix for a beta version is "bN" for some small number N, and the suffix for a release candidate version is "cN" for some small number N. In other words, all versions labeled 2.0aN precede the versions labeled 2.0bN, which precede versions labeled 2.0cN, and those precede 2.0.

You may also find version numbers with a "+" suffix, e.g. "2.2+". These are unreleased versions, built directly from the subversion trunk. In practice, after a final minor release is made, the subversion trunk is incremented to the next minor version, which becomes the "a0" version, e.g. "2.4a0".

See also the documentation for sys.version, sys.hexversion, and sys.version_info.

1.6   How do I obtain a copy of the Python source?

The latest Python source distribution is always available from python.org, at http://www.python.org/download/. The latest development sources can be obtained via anonymous subversion from SourceForge, at http://svn.python.org/projects/python/trunk

The source distribution is a gzipped tar file containing the complete C source, LaTeX documentation, Python library modules, example programs, and several useful pieces of freely distributable software. This will compile and run out of the box on most UNIX platforms.

Older versions of Python are also available from python.org.

1.7   How do I get documentation on Python?

All documentation is available on-line, starting at http://www.python.org/doc/.

The standard documentation for the current stable version of Python is also available at http://docs.python.org/.

The LaTeX source for the documentation is part of the source distribution. If you don't have LaTeX, the latest Python documentation set is available by anonymous FTP in various formats such as PostScript and HTML. Visit the above URL for links to the current versions.

1.8   I've never programmed before. Is there a Python tutorial?

There are numerous tutorials and books available. Consult the Beginner's Guide to find information for beginning Python programmers, including lists of tutorials.

1.10   Is there a newsgroup or mailing list devoted to Python?

There is a newsgroup, comp.lang.python, and a mailing list, python-list. The newsgroup and mailing list are gatewayed into each other -- if you can read news it's unnecessary to subscribe to the mailing list. comp.lang.python is high-traffic, receiving hundreds of postings every day, and Usenet readers are often more able to cope with this volume.

Announcements of new software releases and events can be found in comp.lang.python.announce, a low-traffic moderated list that receives about five postings per day. It's available as the python-announce mailing list.

More info about other mailing lists and newsgroups can be found at http://www.python.org/community/lists.html.

1.11   How do I get a beta test version of Python?

All releases, including alphas, betas and release candidates, are announced on the comp.lang.python and comp.lang.python.announce newsgroups. All announcements also appear on the Python home page, at http://www.python.org/; an RSS feed of news is available.

You can also access the development version of Python through subversion. See http://www.python.org/dev/devfaq.html#subversion-svn for details.

1.12   How do I submit bug reports and patches for Python?

To report a bug or submit a patch, please use the relevant service from the Python project at SourceForge.

Bugs: http://sourceforge.net/tracker/?group_id=5470&atid=105470

Patches: http://sourceforge.net/tracker/?group_id=5470&atid=305470

You must have a SourceForge account to report bugs; this makes it possible for us to contact you if we have follow-up questions. It will also enable SourceForge to send you updates as we act on your bug.

For more information on how Python is developed, consult the Python Developer's Guide.

1.13   Are there any published articles about Python that I can reference?

It's probably best to reference your favorite book about Python.

The very first article about Python is this very old article that's now quite outdated.

Guido van Rossum and Jelke de Boer, "Interactively Testing Remote Servers Using the Python Programming Language", CWI Quarterly, Volume 4, Issue 4 (December 1991), Amsterdam, pp 283-303.

1.14   Are there any books on Python?

Yes, there are many, and more are being published. See the python.org Wiki at http://wiki.python.org/moin/PythonBooks for a list.

You can also search online bookstores for "Python" and filter out the Monty Python references; or perhaps search for "Python" and "language".

1.15   Where in the world is www.python.org located?

It's currently in Amsterdam, graciously hosted by XS4ALL. Thanks to Thomas Wouters for his work in arranging python.org's hosting.

1.16   Why is it called Python?

At the same time he began implementing Python, Guido van Rossum was also reading the published scripts from "Monty Python's Flying Circus" (a BBC comedy series from the seventies, in the unlikely case you didn't know). It occurred to him that he needed a name that was short, unique, and slightly mysterious, so he decided to call the language Python.

2   Python in the real world

2.1   How stable is Python?

Very stable. New, stable releases have been coming out roughly every 6 to 18 months since 1991, and this seems likely to continue. Currently there are usually around 18 months between major releases.

With the introduction of retrospective "bugfix" releases the stability of existing releases is being improved. Bugfix releases, indicated by a third component of the version number (e.g. 2.1.3, 2.2.2), are managed for stability; only fixes for known problems are included in a bugfix release, and it's guaranteed that interfaces will remain the same throughout a series of bugfix releases.

The 2.4.2 release is the most stable version at this point in time.

2.2   How many people are using Python?

Probably tens of thousands of users, though it's difficult to obtain an exact count. Python is available for free download, so there are no sales figures, and it's available from many different sites and packaged with many Linux distributions, so download statistics don't tell the whole story either. The comp.lang.python newsgroup is very active, but not all Python users post to the group or even read it. Overall there is no accurate estimate of the number of subscribers or Python users.

2.3   Have any significant projects been done in Python?

See http://python.org/about/success for a list of projects that use Python. Consulting the proceedings for past Python conferences will reveal contributions from many different companies and organizations.

High-profile Python projects include the Mailman mailing list manager and the Zope application server. Several Linux distributions, most notably Red Hat, have written part or all of their installer and system administration software in Python. Companies that use Python internally include Google, Yahoo, and Industrial Light & Magic.

2.4   What new developments are expected for Python in the future?

See http://www.python.org/doc/peps/ for the Python Enhancement Proposals (PEPs). PEPs are design documents describing a suggested new feature for Python, providing a concise technical specification and a rationale. PEP 1 explains the PEP process and PEP format; read it first if you want to submit a PEP.

New developments are discussed on the python-dev mailing list.

2.5   Is it reasonable to propose incompatible changes to Python?

In general, no. There are already millions of lines of Python code around the world, so any change in the language that invalidates more than a very small fraction of existing programs has to be frowned upon. Even if you can provide a conversion program, there still is the problem of updating all documentation; many books have been written about Python, and we don't want to invalidate them all at a single stroke.

Providing a gradual upgrade path is necessary if a feature has to be changed. PEP 5 describes the procedure followed for introducing backward-incompatible changes while minimizing disruption for users.

2.6   What is the Python Software Foundation?

The Python Software Foundation is an independent non-profit organization that holds the copyright on Python versions 2.1 and newer. The PSF's mission is to advance open source technology related to the Python programming language and to publicize the use of Python. The PSF's home page is at http://www.python.org/psf/.

Donations to the PSF are tax-exempt in the US. If you use Python and find it helpful, please contribute via the PSF donation page.

2.7   Is Python Y2K (Year 2000) Compliant?

As of August, 2003 no major problems have been reported and Y2K compliance seems to be a non-issue.

Python does very few date calculations and for those it does perform relies on the C library functions. Python generally represents times either as seconds since 1970 or as a (year, month, day, ...) tuple where the year is expressed with four digits, which makes Y2K bugs unlikely. So as long as your C library is okay, Python should be okay. Of course, it's possible that a particular application written in Python makes assumptions about 2-digit years.

Because Python is available free of charge, there are no absolute guarantees. If there are unforeseen problems, liability is the user's problem rather than the developers', and there is nobody you can sue for damages. The Python copyright notice contains the following disclaimer:

4. PSF is making Python 2.3 available to Licensee on an "AS IS" basis. PSF MAKES NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED. BY WAY OF EXAMPLE, BUT NOT LIMITATION, PSF MAKES NO AND DISCLAIMS ANY REPRESENTATION OR WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF PYTHON 2.3 WILL NOT INFRINGE ANY THIRD PARTY RIGHTS.

5. PSF SHALL NOT BE LIABLE TO LICENSEE OR ANY OTHER USERS OF PYTHON 2.3 FOR ANY INCIDENTAL, SPECIAL, OR CONSEQUENTIAL DAMAGES OR LOSS AS A RESULT OF MODIFYING, DISTRIBUTING, OR OTHERWISE USING PYTHON 2.3, OR ANY DERIVATIVE THEREOF, EVEN IF ADVISED OF THE POSSIBILITY THEREOF.

The good news is that if you encounter a problem, you have full source available to track it down and fix it. This is one advantage of an open source programming environment.

2.8   Is Python a good language for beginning programmers?

Yes. If you want to discuss Python's use in education, then you may be interested in joining the edu-sig mailing list.

It is still common to start students with a procedural (subset of a) statically typed language such as Pascal, C, or a subset of C++ or Java. Students may be better served by learning Python as their first language. Python has a very simple and consistent syntax and a large standard library and, most importantly, using Python in a beginning programming course permits students to concentrate on important programming skills such as problem decomposition and data type design. With Python, students can be quickly introduced to basic concepts such as loops and procedures. They can even probably work with user-defined objects in their very first course.

For a student who has never programmed before, using a statically typed language seems unnatural. It presents additional complexity that the student must master and slows the pace of the course. The students are trying to learn to think like a computer, decompose problems, design consistent interfaces, and encapsulate data. While learning to use a statically typed language is important in the long term, it is not necessarily the best topic to address in the students' first programming course.

Many other aspects of Python make it a good first language. Like Java, Python has a large standard library so that students can be assigned programming projects very early in the course that do something. Assignments aren't restricted to the standard four-function calculator and check balancing programs. By using the standard library, students can gain the satisfaction of working on realistic applications as they learn the fundamentals of programming. Using the standard library also teaches students about code reuse. Third-party modules such as PyGame are also helpful in extending the students' reach.

Python's interactive interpreter enables students to test language features while they're programming. They can keep a window with the interpreter running while they enter their program's source in another window. If they can't remember the methods for a list, they can do something like this:

>>> L = []
>>> dir(L)
['append', 'count', 'extend', 'index', 'insert', 'pop', 'remove',
'reverse', 'sort']
>>> help(L.append)
Help on built-in function append:

append(...)
    L.append(object) -- append object to end
>>> L.append(1)
>>> L
[1]

With the interpreter, documentation is never far from the student as he's programming.

There are also good IDEs for Python. IDLE is a cross-platform IDE for Python that is written in Python using Tkinter. PythonWin is a Windows-specific IDE. Emacs users will be happy to know that there is a very good Python mode for Emacs. All of these programming environments provide syntax highlighting, auto-indenting, and access to the interactive interpreter while coding. Consult http://www.python.org/editors/ for a full list of Python editing environments.

3   Upgrading Python

3.1   What is this bsddb185 module my application keeps complaining about?

Starting with Python2.3, the distribution includes the PyBSDDB package <http://pybsddb.sf.net/> as a replacement for the old bsddb module. It includes functions which provide backward compatibility at the API level, but requires a newer version of the underlying Berkeley DB library. Files created with the older bsddb module can't be opened directly using the new module.

Using your old version of Python and a pair of scripts which are part of Python 2.3 (db2pickle.py and pickle2db.py, in the Tools/scripts directory) you can convert your old database files to the new format. Using your old Python version, run the db2pickle.py script to convert it to a pickle, e.g.:

python2.2 <pathto>/db2pickley.py database.db database.pck

Rename your database file:

mv database.db olddatabase.db

Now convert the pickle file to a new format database:

python2.3 <pathto>/pickle2db.py database.db database.pck

The precise commands you use will vary depending on the particulars of your installation. For full details about operation of these two scripts check the doc string at the start of each one.

4   Python's Design

4.1   Why does Python use indentation for grouping of statements?

Guido van Rossum believes that using indentation for grouping is extremely elegant and contributes a lot to the clarity of the average Python program. Most people learn to love this feature after awhile.

Since there are no begin/end brackets there cannot be a disagreement between grouping perceived by the parser and the human reader. Occasionally C programmers will encounter a fragment of code like this:

if (x <= y)
        x++;
        y--;
z++;

Only the x++ statement is executed if the condition is true, but the indentation leads you to believe otherwise. Even experienced C programmers will sometimes stare a long time at it wondering why y is being decremented even for x > y.

Because there are no begin/end brackets, Python is much less prone to coding-style conflicts. In C there are many different ways to place the braces. If you're used to reading and writing code that uses one style, you will feel at least slightly uneasy when reading (or being required to write) another style.

Many coding styles place begin/end brackets on a line by themself. This makes programs considerably longer and wastes valuable screen space, making it harder to get a good overview of a program. Ideally, a function should fit on onescreen (say, 20-30 lines). 20 lines of Python can do a lot more work than 20 lines of C. This is not solely due to the lack of begin/end brackets -- the lack of declarations and the high-level data types are also responsible -- but the indentation-based syntax certainly helps.

4.3   Why are floating point calculations so inaccurate?

People are often very surprised by results like this:

>>> 1.2-1.0
0.199999999999999996

and think it is a bug in Python. It's not. It's a problem caused by the internal representation of floating point numbers, which uses a fixed number of binary digits to represent a decimal number. Some decimal numbers can't be represented exactly in binary, resulting in small roundoff errors.

In decimal math, there are many numbers that can't be represented with a fixed number of decimal digits, e.g. 1/3 = 0.3333333333.......

In base 2, 1/2 = 0.1, 1/4 = 0.01, 1/8 = 0.001, etc. .2 equals 2/10 equals 1/5, resulting in the binary fractional number 0.001100110011001...

Floating point numbers only have 32 or 64 bits of precision, so the digits are cut off at some point, and the resulting number is 0.199999999999999996 in decimal, not 0.2.

A floating point's repr() function prints as many digits are necessary to make eval(repr(f)) == f true for any float f. The str() function prints fewer digits and this often results in the more sensible number that was probably intended:

>>> 0.2
0.20000000000000001
>>> print 0.2
0.2

Again, this has nothing to do with Python, but with the way the underlying C platform handles floating point numbers, and ultimately with the inaccuracy you'll always have when writing down numbers as a string of a fixed number of digits.

One of the consequences of this is that it is dangerous to compare the result of some computation to a float with == ! Tiny inaccuracies may mean that == fails. Instead, you have to check that the difference between the two numbers is less than a certain threshold:

epsilon = 0.0000000000001 # Tiny allowed error
expected_result = 0.4

if expected_result-epsilon <= computation() <= expected_result+epsilon:
   ...

Please see the chapter on floating point arithmetic in the Python tutorial for more information.

4.4   Why are Python strings immutable?

There are several advantages.

One is performance: knowing that a string is immutable makes it easy to lay it out at construction time -- fixed and unchanging storage requirements. This is also one of the reasons for the distinction between tuples and lists.

The other is that strings in Python are considered as "elemental" as numbers. No amount of activity will change the value 8 to anything else, and in Python, no amount of activity will change the string "eight" to anything else.

4.5   Why must 'self' be used explicitly in method definitions and calls?

The idea was borrowed from Modula-3. It turns out to be very useful, for a variety of reasons.

First, it's more obvious that you are using a method or instance attribute instead of a local variable. Reading self.x or self.meth() makes it absolutely clear that an instance variable or method is used even if you don't know the class definition by heart. In C++, you can sort of tell by the lack of a local variable declaration (assuming globals are rare or easily recognizable) -- but in Python, there are no local variable declarations, so you'd have to look up the class definition to be sure. Some C++ and Java coding standards call for instance attributes to have an m_ prefix, so this explicitness is still useful in those languages, too.

Second, it means that no special syntax is necessary if you want to explicitly reference or call the method from a particular class. In C++, if you want to use a method from a base class which is overridden in a derived class, you have to use the :: operator -- in Python you can write baseclass.methodname(self, <argument list>). This is particularly useful for __init__() methods, and in general in cases where a derived class method wants to extend the base class method of the same name and thus has to call the base class method somehow.

Finally, for instance variables it solves a syntactic problem with assignment: since local variables in Python are (by definition!) those variables to which a value assigned in a function body (and that aren't explicitly declared global), there has to be some way to tell the interpreter that an assignment was meant to assign to an instance variable instead of to a local variable, and it should preferably be syntactic (for efficiency reasons). C++ does this through declarations, but Python doesn't have declarations and it would be a pity having to introduce them just for this purpose. Using the explicit "self.var" solves this nicely. Similarly, for using instance variables, having to write "self.var" means that references to unqualified names inside a method don't have to search the instance's directories. To put it another way, local variables and instance variables live in two different namespaces, and you need to tell Python which namespace to use.

4.6   Why can't I use an assignment in an expression?

Many people used to C or Perl complain that they want to use this C idiom:

while (line = readline(f)) {
    ...do something with line...
}

where in Python you're forced to write this:

while True:
    line = f.readline()
    if not line:
        break
    ...do something with line...

The reason for not allowing assignment in Python expressions is a common, hard-to-find bug in those other languages, caused by this construct:

if (x = 0) {
    ...error handling...
}
else {
    ...code that only works for nonzero x...
}

The error is a simple typo: x = 0, which assigns 0 to the variable x, was written while the comparison x == 0 is certainly what was intended.

Many alternatives have been proposed. Most are hacks that save some typing but use arbitrary or cryptic syntax or keywords, and fail the simple criterion for language change proposals: it should intuitively suggest the proper meaning to a human reader who has not yet been introduced to the construct.

An interesting phenomenon is that most experienced Python programmers recognize the "while True" idiom and don't seem to be missing the assignment in expression construct much; it's only newcomers who express a strong desire to add this to the language.

There's an alternative way of spelling this that seems attractive but is generally less robust than the "while True" solution:

line = f.readline()
while line:
    ...do something with line...
    line = f.readline()

The problem with this is that if you change your mind about exactly how you get the next line (e.g. you want to change it into sys.stdin.readline()) you have to remember to change two places in your program -- the second occurrence is hidden at the bottom of the loop.

The best approach is to use iterators, making it possible to loop through objects using the for statement. For example, in the current version of Python file objects support the iterator protocol, so you can now write simply:

for line in f:
    ... do something with line...

4.7   Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?

The major reason is history. Functions were used for those operations that were generic for a group of types and which were intended to work even for objects that didn't have methods at all (e.g. tuples). It is also convenient to have a function that can readily be applied to an amorphous collection of objects when you use the functional features of Python (map(), apply() et al).

In fact, implementing len(), max(), min() as a built-in function is actually less code than implementing them as methods for each type. One can quibble about individual cases but it's a part of Python, and it's too late to make such fundamental changes now. The functions have to remain to avoid massive code breakage.

Note that for string operations Python has moved from external functions (the string module) to methods. However, len() is still a function.

4.8   Why is join() a string method instead of a list or tuple method?

Strings became much more like other standard types starting in Python 1.6, when methods were added which give the same functionality that has always been available using the functions of the string module. Most of these new methods have been widely accepted, but the one which appears to make some programmers feel uncomfortable is:

", ".join(['1', '2', '4', '8', '16'])

which gives the result:

"1, 2, 4, 8, 16"

There are two common arguments against this usage.

The first runs along the lines of: "It looks really ugly using a method of a string literal (string constant)", to which the answer is that it might, but a string literal is just a fixed value. If the methods are to be allowed on names bound to strings there is no logical reason to make them unavailable on literals.

The second objection is typically cast as: "I am really telling a sequence to join its members together with a string constant". Sadly, you aren't. For some reason there seems to be much less difficulty with having split() as a string method, since in that case it is easy to see that

"1, 2, 4, 8, 16".split(", ")

is an instruction to a string literal to return the substrings delimited by the given separator (or, by default, arbitrary runs of white space). In this case a Unicode string returns a list of Unicode strings, an ASCII string returns a list of ASCII strings, and everyone is happy.

join() is a string method because in using it you are telling the separator string to iterate over a sequence of strings and insert itself between adjacent elements. This method can be used with any argument which obeys the rules for sequence objects, including any new classes you might define yourself.

Because this is a string method it can work for Unicode strings as well as plain ASCII strings. If join() were a method of the sequence types then the sequence types would have to decide which type of string to return depending on the type of the separator.

If none of these arguments persuade you, then for the moment you can continue to use the join() function from the string module, which allows you to write

string.join(['1', '2', '4', '8', '16'], ", ")

4.9   How fast are exceptions?

A try/except block is extremely efficient. Actually executing an exception is expensive. In versions of Python prior to 2.0 it was common to use this idiom:

try:
    value = dict[key]
except KeyError:
    dict[key] = getvalue(key)
    value = dict[key]

This only made sense when you expected the dict to have the key almost all the time. If that wasn't the case, you coded it like this:

if dict.has_key(key):
    value = dict[key]
else:
    dict[key] = getvalue(key)
    value = dict[key]

(In Python 2.0 and higher, you can code this as value = dict.setdefault(key, getvalue(key)).)

4.10   Why isn't there a switch or case statement in Python?

You can do this easily enough with a sequence of if... elif... elif... else. There have been some proposals for switch statement syntax, but there is no consensus (yet) on whether and how to do range tests. See PEP 275 for complete details and the current status.

For cases where you need to choose from a very large number of possibilities, you can create a dictionary mapping case values to functions to call. For example:

def function_1 (...):
    ...

functions = {'a': function_1,
             'b': function_2,
             'c': self.method_1, ...}

func = functions[value]
func()

For calling methods on objects, you can simplify yet further by using the getattr() built-in to retrieve methods with a particular name:

def visit_a (self, ...):
    ...
...

def dispatch (self, value):
    method_name = 'visit_' + str(value)
    method = getattr(self, method_name)
    method()

It's suggested that you use a prefix for the method names, such as visit_ in this example. Without such a prefix, if values are coming from an untrusted source, an attacker would be able to call any method on your object.

4.11   Can't you emulate threads in the interpreter instead of relying on an OS-specific thread implementation?

Answer 1: Unfortunately, the interpreter pushes at least one C stack frame for each Python stack frame. Also, extensions can call back into Python at almost random moments. Therefore, a complete threads implementation requires thread support for C.

Answer 2: Fortunately, there is Stackless Python, which has a completely redesigned interpreter loop that avoids the C stack. It's still experimental but looks very promising. Although it is binary compatible with standard Python, it's still unclear whether Stackless will make it into the core -- maybe it's just too revolutionary.

4.12   Why can't lambda forms contain statements?

Python lambda forms cannot contain statements because Python's syntactic framework can't handle statements nested inside expressions. However, in Python, this is not a serious problem. Unlike lambda forms in other languages, where they add functionality, Python lambdas are only a shorthand notation if you're too lazy to define a function.

Functions are already first class objects in Python, and can be declared in a local scope. Therefore the only advantage of using a lambda form instead of a locally-defined function is that you don't need to invent a name for the function -- but that's just a local variable to which the function object (which is exactly the same type of object that a lambda form yields) is assigned!

4.13   Can Python be compiled to machine code, C or some other language?

Not easily. Python's high level data types, dynamic typing of objects and run-time invocation of the interpreter (using eval() or exec) together mean that a "compiled" Python program would probably consist mostly of calls into the Python run-time system, even for seemingly simple operations like x+1.

Several projects described in the Python newsgroup or at past Python conferences have shown that this approach is feasible, although the speedups reached so far are only modest (e.g. 2x). Jython uses the same strategy for compiling to Java bytecode. (Jim Hugunin has demonstrated that in combination with whole-program analysis, speedups of 1000x are feasible for small demo programs. See the proceedings from the 1997 Python conference for more information.)

Internally, Python source code is always translated into a bytecode representation, and this bytecode is then executed by the Python virtual machine. In order to avoid the overhead of repeatedly parsing and translating modules that rarely change, this byte code is written into a file whose name ends in ".pyc" whenever a module is parsed. When the corresponding .py file is changed, it is parsed and translated again and the .pyc file is rewritten.

There is no performance difference once the .pyc file has been loaded, as the bytecode read from the .pyc file is exactly the same as the bytecode created by direct translation. The only difference is that loading code from a .pyc file is faster than parsing and translating a .py file, so the presence of precompiled .pyc files improves the start-up time of Python scripts. If desired, the Lib/compileall.py module can be used to create valid .pyc files for a given set of modules.

Note that the main script executed by Python, even if its filename ends in .py, is not compiled to a .pyc file. It is compiled to bytecode, but the bytecode is not saved to a file. Usually main scripts are quite short, so this doesn't cost much speed.

There are also several programs which make it easier to intermingle Python and C code in various ways to increase performance. See, for example, Psyco, Pyrex, PyInline, Py2Cmod, and Weave.

4.14   How does Python manage memory?

The details of Python memory management depend on the implementation. The standard C implementation of Python uses reference counting to detect inaccessible objects, and another mechanism to collect reference cycles, periodically executing a cycle detection algorithm which looks for inaccessible cycles and deletes the objects involved. The gc module provides functions to perform a garbage collection, obtain debugging statistics, and tune the collector's parameters.

Jython relies on the Java runtime so the JVM's garbage collector is used. This difference can cause some subtle porting problems if your Python code depends on the behavior of the reference counting implementation.

Sometimes objects get stuck in tracebacks temporarily and hence are not deallocated when you might expect. Clear the tracebacks with:

import sys
sys.exc_clear()
sys.exc_traceback = sys.last_traceback = None

Tracebacks are used for reporting errors, implementing debuggers and related things. They contain a portion of the program state extracted during the handling of an exception (usually the most recent exception).

In the absence of circularities and tracebacks, Python programs need not explicitly manage memory.

Why doesn't Python use a more traditional garbage collection scheme? For one thing, this is not a C standard feature and hence it's not portable. (Yes, we know about the Boehm GC library. It has bits of assembler code for most common platforms, not for all of them, and although it is mostly transparent, it isn't completely transparent; patches are required to get Python to work with it.)

Traditional GC also becomes a problem when Python is embedded into other applications. While in a standalone Python it's fine to replace the standard malloc() and free() with versions provided by the GC library, an application embedding Python may want to have its own substitute for malloc() and free(), and may not want Python's. Right now, Python works with anything that implements malloc() and free() properly.

In Jython, the following code (which is fine in CPython) will probably run out of file descriptors long before it runs out of memory:

for file in <very long list of files>:
    f = open(file)
    c = f.read(1)

Using the current reference counting and destructor scheme, each new assignment to f closes the previous file. Using GC, this is not guaranteed. If you want to write code that will work with any Python implementation, you should explicitly close the file; this will work regardless of GC:

for file in <very long list of files>:
    f = open(file)
    c = f.read(1)
    f.close()

4.15   Why isn't all memory freed when Python exits?

Objects referenced from the global namespaces of Python modules are not always deallocated when Python exits. This may happen if there are circular references. There are also certain bits of memory that are allocated by the C library that are impossible to free (e.g. a tool like Purify will complain about these). Python is, however, aggressive about cleaning up memory on exit and does try to destroy every single object.

If you want to force Python to delete certain things on deallocation use the sys.exitfunc() hook to run a function that will force those deletions.

4.16   Why are there separate tuple and list data types?

Lists and tuples, while similar in many respects, are generally used in fundamentally different ways. Tuples can be thought of as being similar to Pascal records or C structs; they're small collections of related data which may be of different types which are operated on as a group. For example, a Cartesian coordinate is appropriately represented as a tuple of two or three numbers.

Lists, on the other hand, are more like arrays in other languages. They tend to hold a varying number of objects all of which have the same type and which are operated on one-by-one. For example, os.listdir('.') returns a list of strings representing the files in the current directory. Functions which operate on this output would generally not break if you added another file or two to the directory.

Tuples are immutable, meaning that once a tuple has been created, you can't replace any of its elements with a new value. Lists are mutable, meaning that you can always change a list's elements. Only immutable elements can be used as dictionary keys, and hence only tuples and not lists can be used as keys.

4.17   How are lists implemented?

Python's lists are really variable-length arrays, not Lisp-style linked lists. The implementation uses a contiguous array of references to other objects, and keeps a pointer to this array and the array's length in a list head structure.

This makes indexing a list a[i] an operation whose cost is independent of the size of the list or the value of the index.

When items are appended or inserted, the array of references is resized. Some cleverness is applied to improve the performance of appending items repeatedly; when the array must be grown, some extra space is allocated so the next few times don't require an actual resize.

4.18   How are dictionaries implemented?

Python's dictionaries are implemented as resizable hash tables. Compared to B-trees, this gives better performance for lookup (the most common operation by far) under most circumstances, and the implementation is simpler.

Dictionaries work by computing a hash code for each key stored in the dictionary using the hash() built-in function. The hash code varies widely depending on the key; for example, "Python" hashes to -539294296 while "python", a string that differs by a single bit, hashes to 1142331976. The hash code is then used to calculate a location in an internal array where the value will be stored. Assuming that you're storing keys that all have different hash values, this means that dictionaries take constant time -- O(1), in computer science notation -- to retrieve a key. It also means that no sorted order of the keys is maintained, and traversing the array as the .keys() and .items() do will output the dictionary's content in some arbitrary jumbled order.

4.19   Why must dictionary keys be immutable?

The hash table implementation of dictionaries uses a hash value calculated from the key value to find the key. If the key were a mutable object, its value could change, and thus its hash could also change. But since whoever changes the key object can't tell that it was being used as a dictionary key, it can't move the entry around in the dictionary. Then, when you try to look up the same object in the dictionary it won't be found because its hash value is different. If you tried to look up the old value it wouldn't be found either, because the value of the object found in that hash bin would be different.

If you want a dictionary indexed with a list, simply convert the list to a tuple first; the function tuple(L) creates a tuple with the same entries as the list L. Tuples are immutable and can therefore be used as dictionary keys.

Some unacceptable solutions that have been proposed:

  • Hash lists by their address (object ID). This doesn't work because if you construct a new list with the same value it won't be found; e.g.:

    d = {[1,2]: '12'}
    print d[[1,2]]
    

    would raise a KeyError exception because the id of the [1,2] used in the second line differs from that in the first line. In other words, dictionary keys should be compared using ==, not using 'is'.

  • Make a copy when using a list as a key. This doesn't work because the list, being a mutable object, could contain a reference to itself, and then the copying code would run into an infinite loop.

  • Allow lists as keys but tell the user not to modify them. This would allow a class of hard-to-track bugs in programs when you forgot or modified a list by accident. It also invalidates an important invariant of dictionaries: every value in d.keys() is usable as a key of the dictionary.

  • Mark lists as read-only once they are used as a dictionary key. The problem is that it's not just the top-level object that could change its value; you could use a tuple containing a list as a key. Entering anything as a key into a dictionary would require marking all objects reachable from there as read-only -- and again, self-referential objects could cause an infinite loop.

There is a trick to get around this if you need to, but use it at your own risk: You can wrap a mutable structure inside a class instance which has both a __cmp__ and a __hash__ method. You must then make sure that the hash value for all such wrapper objects that reside in a dictionary (or other hash based structure), remain fixed while the object is in the dictionary (or other structure).:

class ListWrapper:
     def __init__(self, the_list):
           self.the_list = the_list
     def __cmp__(self, other):
           return self.the_list == other.the_list
     def __hash__(self):
           l = self.the_list
           result = 98767 - len(l)*555
           for i in range(len(l)):
                try:
                     result = result + (hash(l[i]) % 9999999) * 1001 + i
                except:
                     result = (result % 7777777) + i * 333
           return result

Note that the hash computation is complicated by the possibility that some members of the list may be unhashable and also by the possibility of arithmetic overflow.

Furthermore it must always be the case that if o1 == o2 (ie o1.__cmp__(o2)==0) then hash(o1)==hash(o2) (ie, o1.__hash__() == o2.__hash__()), regardless of whether the object is in a dictionary or not. If you fail to meet these restrictions dictionaries and other hash based structures will misbehave.

In the case of ListWrapper, whenever the wrapper object is in a dictionary the wrapped list must not change to avoid anomalies. Don't do this unless you are prepared to think hard about the requirements and the consequences of not meeting them correctly. Consider yourself warned.

4.20   Why doesn't list.sort() return the sorted list?

In situations where performance matters, making a copy of the list just to sort it would be wasteful. Therefore, list.sort() sorts the list in place. In order to remind you of that fact, it does not return the sorted list. This way, you won't be fooled into accidentally overwriting a list when you need a sorted copy but also need to keep the unsorted version around.

In Python 2.4 a new builtin - sorted() - has been added. This function creates a new list from a passed iterable, sorts it and returns it.

As a result, here's the idiom to iterate over the keys of a dictionary in sorted order:

for key in sorted(dict.iterkeys()):
    ...do whatever with dict[key]...

Versions of Python prior to 2.4 need to use the following idiom:

keys = dict.keys()
keys.sort()
for key in keys:
    ...do whatever with dict[key]...

4.21   How do you specify and enforce an interface spec in Python?

An interface specification for a module as provided by languages such as C++ and Java describes the prototypes for the methods and functions of the module. Many feel that compile-time enforcement of interface specifications help in the construction of large programs. Python does not support interface specifications directly, but many of their advantages can be obtained by an appropriate test discipline for components, which can often be very easily accomplished in Python. There is also a tool, PyChecker, which can be used to find problems due to subclassing.

A good test suite for a module can at once provide a regression test and serve as both a module interface specification and a set of examples. Many Python modules can be run as a script to provide a simple "self test." Even modules which use complex external interfaces can often be tested in isolation using trivial "stub" emulations of the external interface. The doctest and unittest modules or third-party test frameworks can be used to construct exhaustive test suites that exercise every line of code in a module.

An appropriate testing discipline can help build large complex applications in Python as well as having interface specifications would. In fact, it can be better because an interface specification cannot test certain properties of a program. For example, the append() method is expected to add new elements to the end of some internal list; an interface specification cannot test that your append() implementation will actually do this correctly, but it's trivial to check this property in a test suite.

Writing test suites is very helpful, and you might want to design your code with an eye to making it easily tested. One increasingly popular technique, test-directed development, calls for writing parts of the test suite first, before you write any of the actual code. Of course Python allows you to be sloppy and not write test cases at all.

4.22   Why are default values shared between objects?

This type of bug commonly bites neophyte programmers. Consider this function:

def foo(D={}):  # Danger: shared reference to one dict for all calls
    ... compute something ...
    D[key] = value
    return D

The first time you call this function, D contains a single item. The second time, D contains two items because when foo() begins executing, D starts out with an item already in it.

It is often expected that a function call creates new objects for default values. This is not what happens. Default values are created exactly once, when the function is defined. If that object is changed, like the dictionary in this example, subsequent calls to the function will refer to this changed object.

By definition, immutable objects such as numbers, strings, tuples, and None, are safe from change. Changes to mutable objects such as dictionaries, lists, and class instances can lead to confusion.

Because of this feature, it is good programming practice to not use mutable objects as default values. Instead, use None as the default value and inside the function, check if the parameter is None and create a new list/dictionary/whatever if it is. For example, don't write:

def foo(dict={}):
    ...

but:

def foo(dict=None):
    if dict is None:
        dict = {} # create a new dict for local namespace

This feature can be useful. When you have a function that's time-consuming to compute, a common technique is to cache the parameters and the resulting value of each call to the function, and return the cached value if the same value is requested again. This is called "memoizing", and can be implemented like this:

# Callers will never provide a third parameter for this function.
def expensive (arg1, arg2, _cache={}):
    if _cache.has_key((arg1, arg2)):
        return _cache[(arg1, arg2)]

    # Calculate the value
    result = ... expensive computation ...
    _cache[(arg1, arg2)] = result           # Store result in the cache
    return result

You could use a global variable containing a dictionary instead of the default value; it's a matter of taste.

4.23   Why is there no goto?

You can use exceptions to provide a "structured goto" that even works across function calls. Many feel that exceptions can conveniently emulate all reasonable uses of the "go" or "goto" constructs of C, Fortran, and other languages. For example:

class label: pass # declare a label

try:
     ...
     if (condition): raise label() # goto label
     ...
except label: # where to goto
     pass
...

This doesn't allow you to jump into the middle of a loop, but that's usually considered an abuse of goto anyway. Use sparingly.

4.24   Why do I get a SyntaxError for a 'continue' inside a 'try'?

This is an implementation limitation, caused by the extremely simple-minded way Python generates bytecode. The try block pushes something on the "block stack" which the continue would have to pop off again. The current code generator doesn't have the data structures around so that continue can generate the right code.

Note that Jython doesn't have this restriction!

4.25   Why can't raw strings (r-strings) end with a backslash?

More precisely, they can't end with an odd number of backslashes: the unpaired backslash at the end escapes the closing quote character, leaving an unterminated string.

Raw strings were designed to ease creating input for processors (chiefly regular expression engines) that want to do their own backslash escape processing. Such processors consider an unmatched trailing backslash to be an error anyway, so raw strings disallow that. In return, they allow you to pass on the string quote character by escaping it with a backslash. These rules work well when r-strings are used for their intended purpose.

If you're trying to build Windows pathnames, note that all Windows system calls accept forward slashes too:

f = open("/mydir/file.txt") # works fine!

If you're trying to build a pathname for a DOS command, try e.g. one of

dir = r"\this\is\my\dos\dir" "\\"
dir = r"\this\is\my\dos\dir\ "[:-1]
dir = "\\this\\is\\my\\dos\\dir\\"

4.26   Why doesn't Python have a "with" statement like some other languages?

Because such a construct would be ambiguous.

Some languages, such as Object Pascal, Delphi, and C++, use static types. So it is possible to know, in an unambiguous way, what member is being assigned in a "with" clause. This is the main point - the compiler always knows the scope of every variable at compile time.

Python uses dynamic types. It is impossible to know in advance which attribute will be referenced at runtime. Member attributes may be added or removed from objects on the fly. This would make it impossible to know, from a simple reading, what attribute is being referenced - a local one, a global one, or a member attribute.

For instance, take the following incomplete snippet:

def foo(a):
   with a:
      print x

The snippet assumes that "a" must have a member attribute called "x". However, there is nothing in Python that guarantees that. What should happen if "a" is, let us say, an integer? And if I have a global variable named "x", will it end up being used inside the with block? As you see, the dynamic nature of Python makes such choices much harder.

The primary benefit of "with" and similar language features (reduction of code volume) can, however, easily be achieved in Python by assignment. Instead of:

function(args).dict[index][index].a = 21
function(args).dict[index][index].b = 42
function(args).dict[index][index].c = 63

write this:

ref = function(args).dict[index][index]
ref.a = 21
ref.b = 42
ref.c = 63

This also has the side-effect of increasing execution speed because name bindings are resolved at run-time in Python, and the second version only needs to perform the resolution once. If the referenced object does not have a, b and c attributes, of course, the end result is still a run-time exception.

4.27   Why are colons required for the if/while/def/class statements?

The colon is required primarily to enhance readability (one of the results of the experimental ABC language). Consider this:

if a==b
    print a

versus

if a==b:
    print a

Notice how the second one is slightly easier to read. Notice further how a colon sets off the example in the second line of this FAQ answer; it's a standard usage in English.

Another minor reason is that the colon makes it easier for editors with syntax highlighting; they can look for colons to decide when indentation needs to be increased instead of having to do a more elaborate parsing of the program text.