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Chapter 2. Your First Python Program
- 2.1. Diving in
In the ActivePython IDE on Windows, you can run the Python program you're editing by choosing -> (Ctrl-R). Output is displayed in the interactive window. In the Python IDE on Mac OS, you can run a Python program with -> (Cmd-R), but there is an important option you must set first. Open the .py file in the IDE, pop up the options menu by clicking the black triangle in the upper-right corner of the window, and make sure the option is checked. This is a per-file setting, but you'll only need to do it once per file. On UNIX-compatible systems (including Mac OS X), you can run a Python program from the command line: python odbchelper.py - 2.2. Declaring Functions
In Visual Basic, functions (that return a value) start with function, and subroutines (that do not return a value) start with sub. There are no subroutines in Python. Everything is a function, all functions return a value (even if it's None), and all functions start with def. In Java, C++, and other statically-typed languages, you must specify the datatype of the function return value and each function argument. In Python, you never explicitly specify the datatype of anything. Based on what value you assign, Python keeps track of the datatype internally. - 2.3. Documenting Functions
Triple quotes are also an easy way to define a string with both single and double quotes, like qq/.../ in Perl. Many Python IDEs use the doc string to provide context-sensitive documentation, so that when you type a function name, its doc string appears as a tooltip. This can be incredibly helpful, but it's only as good as the doc strings you write. - 2.4. Everything Is an Object
import in Python is like require in Perl. Once you import a Python module, you access its functions with module.function; once you require a Perl module, you access its functions with module::function. - 2.5. Indenting Code
Python uses carriage returns to separate statements and a colon and indentation to separate code blocks. C++ and Java use semicolons to separate statements and curly braces to separate code blocks. - 2.6. Testing Modules
Like C, Python uses == for comparison and = for assignment. Unlike C, Python does not support in-line assignment, so there's no chance of accidentally assigning the value you thought you were comparing. On MacPython, there is an additional step to make the if __name__ trick work. Pop up the module's options menu by clicking the black triangle in the upper-right corner of the window, and make sure is checked.
- 3.1. Introducing Dictionaries
A dictionary in Python is like a hash in Perl. In Perl, variables that store hashes always start with a % character. In Python, variables can be named anything, and Python keeps track of the datatype internally. A dictionary in Python is like an instance of the Hashtable class in Java. A dictionary in Python is like an instance of the Scripting.Dictionary object in Visual Basic. - 3.1.2. Modifying Dictionaries
Dictionaries have no concept of order among elements. It is incorrect to say that the elements are “out of order”; they are simply unordered. This is an important distinction that will annoy you when you want to access the elements of a dictionary in a specific, repeatable order (like alphabetical order by key). There are ways of doing this, but they're not built into the dictionary. - 3.2. Introducing Lists
A list in Python is like an array in Perl. In Perl, variables that store arrays always start with the @ character; in Python, variables can be named anything, and Python keeps track of the datatype internally. A list in Python is much more than an array in Java (although it can be used as one if that's really all you want out of life). A better analogy would be to the ArrayList class, which can hold arbitrary objects and can expand dynamically as new items are added. - 3.2.3. Searching Lists
Before version 2.2.1, Python had no separate boolean datatype. To compensate for this, Python accepted almost anything in a boolean context (like an if statement), according to the following rules: These rules still apply in Python 2.2.1 and beyond, but now you can also use an actual boolean, which has a value of True or False. Note the capitalization; these values, like everything else in Python, are case-sensitive.- 0 is false; all other numbers are true.
- An empty string ("") is false, all other strings are true.
- An empty list ([]) is false; all other lists are true.
- An empty tuple (()) is false; all other tuples are true.
- An empty dictionary ({}) is false; all other dictionaries are true.
- 3.3. Introducing Tuples
Tuples can be converted into lists, and vice-versa. The built-in tuple function takes a list and returns a tuple with the same elements, and the list function takes a tuple and returns a list. In effect, tuple freezes a list, and list thaws a tuple. - 3.4. Declaring variables
When a command is split among several lines with the line-continuation marker (“\”), the continued lines can be indented in any manner; Python's normally stringent indentation rules do not apply. If your Python IDE auto-indents the continued line, you should probably accept its default unless you have a burning reason not to. - 3.5. Formatting Strings
String formatting in Python uses the same syntax as the sprintf function in C. - 3.7. Joining Lists and Splitting Strings
join works only on lists of strings; it does not do any type coercion. Joining a list that has one or more non-string elements will raise an exception. anystring.split(delimiter, 1) is a useful technique when you want to search a string for a substring and then work with everything before the substring (which ends up in the first element of the returned list) and everything after it (which ends up in the second element).
Chapter 4. The Power Of Introspection
- 4.2. Using Optional and Named Arguments
The only thing you need to do to call a function is specify a value (somehow) for each required argument; the manner and order in which you do that is up to you. - 4.3.3. Built-In Functions
Python comes with excellent reference manuals, which you should peruse thoroughly to learn all the modules Python has to offer. But unlike most languages, where you would find yourself referring back to the manuals or man pages to remind yourself how to use these modules, Python is largely self-documenting. - 4.7. Using lambda Functions
lambda functions are a matter of style. Using them is never required; anywhere you could use them, you could define a separate normal function and use that instead. I use them in places where I want to encapsulate specific, non-reusable code without littering my code with a lot of little one-line functions. - 4.8. Putting It All Together
In SQL, you must use IS NULL instead of = NULL to compare a null value. In Python, you can use either == None or is None, but is None is faster.
Chapter 5. Objects and Object-Orientation
- 5.2. Importing Modules Using from module import
from module import * in Python is like use module in Perl; import module in Python is like require module in Perl. from module import * in Python is like import module.* in Java; import module in Python is like import module in Java. Use from module import * sparingly, because it makes it difficult to determine where a particular function or attribute came from, and that makes debugging and refactoring more difficult. - 5.3. Defining Classes
The pass statement in Python is like an empty set of braces ({}) in Java or C. In Python, the ancestor of a class is simply listed in parentheses immediately after the class name. There is no special keyword like extends in Java. - 5.3.1. Initializing and Coding Classes
By convention, the first argument of any Python class method (the reference to the current instance) is called self. This argument fills the role of the reserved word this in C++ or Java, but self is not a reserved word in Python, merely a naming convention. Nonetheless, please don't call it anything but self; this is a very strong convention. - 5.3.2. Knowing When to Use self and __init__
__init__ methods are optional, but when you define one, you must remember to explicitly call the ancestor's __init__ method (if it defines one). This is more generally true: whenever a descendant wants to extend the behavior of the ancestor, the descendant method must explicitly call the ancestor method at the proper time, with the proper arguments. - 5.4. Instantiating Classes
In Python, simply call a class as if it were a function to create a new instance of the class. There is no explicit new operator like C++ or Java. - 5.5. Exploring UserDict: A Wrapper Class
In the ActivePython IDE on Windows, you can quickly open any module in your library path by selecting -> (Ctrl-L). Java and Powerbuilder support function overloading by argument list, i.e. one class can have multiple methods with the same name but a different number of arguments, or arguments of different types. Other languages (most notably PL/SQL) even support function overloading by argument name; i.e. one class can have multiple methods with the same name and the same number of arguments of the same type but different argument names. Python supports neither of these; it has no form of function overloading whatsoever. Methods are defined solely by their name, and there can be only one method per class with a given name. So if a descendant class has an __init__ method, it always overrides the ancestor __init__ method, even if the descendant defines it with a different argument list. And the same rule applies to any other method. Guido, the original author of Python, explains method overriding this way: "Derived classes may override methods of their base classes. Because methods have no special privileges when calling other methods of the same object, a method of a base class that calls another method defined in the same base class, may in fact end up calling a method of a derived class that overrides it. (For C++ programmers: all methods in Python are effectively virtual.)" If that doesn't make sense to you (it confuses the hell out of me), feel free to ignore it. I just thought I'd pass it along. Always assign an initial value to all of an instance's data attributes in the __init__ method. It will save you hours of debugging later, tracking down AttributeError exceptions because you're referencing uninitialized (and therefore non-existent) attributes. In versions of Python prior to 2.2, you could not directly subclass built-in datatypes like strings, lists, and dictionaries. To compensate for this, Python comes with wrapper classes that mimic the behavior of these built-in datatypes: UserString, UserList, and UserDict. Using a combination of normal and special methods, the UserDict class does an excellent imitation of a dictionary. In Python 2.2 and later, you can inherit classes directly from built-in datatypes like dict. An example of this is given in the examples that come with this book, in fileinfo_fromdict.py. - 5.6.1. Getting and Setting Items
When accessing data attributes within a class, you need to qualify the attribute name: self.attribute. When calling other methods within a class, you need to qualify the method name: self.method. - 5.7. Advanced Special Class Methods
In Java, you determine whether two string variables reference the same physical memory location by using str1 == str2. This is called object identity, and it is written in Python as str1 is str2. To compare string values in Java, you would use str1.equals(str2); in Python, you would use str1 == str2. Java programmers who have been taught to believe that the world is a better place because == in Java compares by identity instead of by value may have a difficult time adjusting to Python's lack of such “gotchas”. While other object-oriented languages only let you define the physical model of an object (“this object has a GetLength method”), Python's special class methods like __len__ allow you to define the logical model of an object (“this object has a length”). - 5.8. Introducing Class Attributes
In Java, both static variables (called class attributes in Python) and instance variables (called data attributes in Python) are defined immediately after the class definition (one with the static keyword, one without). In Python, only class attributes can be defined here; data attributes are defined in the __init__ method. There are no constants in Python. Everything can be changed if you try hard enough. This fits with one of the core principles of Python: bad behavior should be discouraged but not banned. If you really want to change the value of None, you can do it, but don't come running to me when your code is impossible to debug. - 5.9. Private Functions
In Python, all special methods (like __setitem__) and built-in attributes (like __doc__) follow a standard naming convention: they both start with and end with two underscores. Don't name your own methods and attributes this way, because it will only confuse you (and others) later.
Chapter 6. Exceptions and File Handling
- 6.1. Handling Exceptions
Python uses try...except to handle exceptions and raise to generate them. Java and C++ use try...catch to handle exceptions, and throw to generate them. - 6.5. Working with Directories
Whenever possible, you should use the functions in os and os.path for file, directory, and path manipulations. These modules are wrappers for platform-specific modules, so functions like os.path.split work on UNIX, Windows, Mac OS, and any other platform supported by Python.
Chapter 7. Regular Expressions
- 7.4. Using the {n,m} Syntax
There is no way to programmatically determine that two regular expressions are equivalent. The best you can do is write a lot of test cases to make sure they behave the same way on all relevant inputs. You'll talk more about writing test cases later in this book.
- 8.2. Introducing sgmllib.py
Python 2.0 had a bug where SGMLParser would not recognize declarations at all (handle_decl would never be called), which meant that DOCTYPEs were silently ignored. This is fixed in Python 2.1. In the ActivePython IDE on Windows, you can specify command line arguments in the “Run script” dialog. Separate multiple arguments with spaces. - 8.4. Introducing BaseHTMLProcessor.py
The HTML specification requires that all non-HTML (like client-side JavaScript) must be enclosed in HTML comments, but not all web pages do this properly (and all modern web browsers are forgiving if they don't). BaseHTMLProcessor is not forgiving; if script is improperly embedded, it will be parsed as if it were HTML. For instance, if the script contains less-than and equals signs, SGMLParser may incorrectly think that it has found tags and attributes. SGMLParser always converts tags and attribute names to lowercase, which may break the script, and BaseHTMLProcessor always encloses attribute values in double quotes (even if the original HTML document used single quotes or no quotes), which will certainly break the script. Always protect your client-side script within HTML comments. - 8.5. locals and globals
Python 2.2 introduced a subtle but important change that affects the namespace search order: nested scopes. In versions of Python prior to 2.2, when you reference a variable within a nested function or lambda function, Python will search for that variable in the current (nested or lambda) function's namespace, then in the module's namespace. Python 2.2 will search for the variable in the current (nested or lambda) function's namespace, then in the parent function's namespace, then in the module's namespace. Python 2.1 can work either way; by default, it works like Python 2.0, but you can add the following line of code at the top of your module to make your module work like Python 2.2: from __future__ import nested_scopes
Using the locals and globals functions, you can get the value of arbitrary variables dynamically, providing the variable name as a string. This mirrors the functionality of the getattr function, which allows you to access arbitrary functions dynamically by providing the function name as a string. - 8.6. Dictionary-based string formatting
Using dictionary-based string formatting with locals is a convenient way of making complex string formatting expressions more readable, but it comes with a price. There is a slight performance hit in making the call to locals, since locals builds a copy of the local namespace.
- 9.2. Packages
A package is a directory with the special __init__.py file in it. The __init__.py file defines the attributes and methods of the package. It doesn't need to define anything; it can just be an empty file, but it has to exist. But if __init__.py doesn't exist, the directory is just a directory, not a package, and it can't be imported or contain modules or nested packages. - 9.6. Accessing element attributes
This section may be a little confusing, because of some overlapping terminology. Elements in an XML document have attributes, and Python objects also have attributes. When you parse an XML document, you get a bunch of Python objects that represent all the pieces of the XML document, and some of these Python objects represent attributes of the XML elements. But the (Python) objects that represent the (XML) attributes also have (Python) attributes, which are used to access various parts of the (XML) attribute that the object represents. I told you it was confusing. I am open to suggestions on how to distinguish these more clearly. Like a dictionary, attributes of an XML element have no ordering. Attributes may happen to be listed in a certain order in the original XML document, and the Attr objects may happen to be listed in a certain order when the XML document is parsed into Python objects, but these orders are arbitrary and should carry no special meaning. You should always access individual attributes by name, like the keys of a dictionary.
Chapter 10. Scripts and Streams
- 11.6. Handling Last-Modified and ETag
In these examples, the HTTP server has supported both Last-Modified and ETag headers, but not all servers do. As a web services client, you should be prepared to support both, but you must code defensively in case a server only supports one or the other, or neither.
- 13.2. Diving in
unittest is included with Python 2.1 and later. Python 2.0 users can download it from pyunit.sourceforge.net.
Chapter 14. Test-First Programming
- 14.3. roman.py, stage 3
The most important thing that comprehensive unit testing can tell you is when to stop coding. When all the unit tests for a function pass, stop coding the function. When all the unit tests for an entire module pass, stop coding the module. - 14.5. roman.py, stage 5
When all of your tests pass, stop coding.
- 15.3. Refactoring
Whenever you are going to use a regular expression more than once, you should compile it to get a pattern object, then call the methods on the pattern object directly.
Chapter 16. Functional Programming
- 16.2. Finding the path
The pathnames and filenames you pass to os.path.abspath do not need to exist. os.path.abspath not only constructs full path names, it also normalizes them. That means that if you are in the /usr/ directory, os.path.abspath('bin/../local/bin') will return /usr/local/bin. It normalizes the path by making it as simple as possible. If you just want to normalize a pathname like this without turning it into a full pathname, use os.path.normpath instead. Like the other functions in the os and os.path modules, os.path.abspath is cross-platform. Your results will look slightly different than my examples if you're running on Windows (which uses backslash as a path separator) or Mac OS (which uses colons), but they'll still work. That's the whole point of the os module.
Chapter 18. Performance Tuning
- 18.2. Using the timeit Module
You can use the timeit module on the command line to test an existing Python program, without modifying the code. See http://docs.python.org/lib/node396.html for documentation on the command-line flags. The timeit module only works if you already know what piece of code you need to optimize. If you have a larger Python program and don't know where your performance problems are, check out the hotshot module.
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