Memory-mapped file objects behave like both
bytearray and like
file objects. You can use mmap objects in most places
bytearray are expected; for example, you can use the
module to search through a memory-mapped file. You can also change a single
byte by doing
obj[index] = 97, or change a subsequence by assigning to a
obj[i1:i2] = b'...'. You can also read and write data starting at
the current file position, and
seek() through the file to different positions.
A memory-mapped file is created by the
mmap constructor, which is
different on Unix and on Windows. In either case you must provide a file
descriptor for a file opened for update. If you wish to map an existing Python
file object, use its
fileno() method to obtain the correct value for the
fileno parameter. Otherwise, you can open the file using the
os.open() function, which returns a file descriptor directly (the file
still needs to be closed when done).
If you want to create a memory-mapping for a writable, buffered file, you
flush() the file first. This is necessary to ensure
that local modifications to the buffers are actually available to the
For both the Unix and Windows versions of the constructor, access may be
specified as an optional keyword parameter. access accepts one of three
to specify read-only, write-through or copy-on-write memory respectively.
access can be used on both Unix and Windows. If access is not specified,
Windows mmap returns a write-through mapping. The initial memory values for
all three access types are taken from the specified file. Assignment to an
ACCESS_READ memory map raises a
Assignment to an
ACCESS_WRITE memory map affects both memory and the
underlying file. Assignment to an
ACCESS_COPY memory map affects
memory but does not update the underlying file.
To map anonymous memory, -1 should be passed as the fileno along with the length.
mmap(fileno, length, tagname=None, access=ACCESS_DEFAULT[, offset])¶
(Windows version) Maps length bytes from the file specified by the file handle fileno, and creates a mmap object. If length is larger than the current size of the file, the file is extended to contain length bytes. If length is
0, the maximum length of the map is the current size of the file, except that if the file is empty Windows raises an exception (you cannot create an empty mapping on Windows).
tagname, if specified and not
None, is a string giving a tag name for the mapping. Windows allows you to have many different mappings against the same file. If you specify the name of an existing tag, that tag is opened, otherwise a new tag of this name is created. If this parameter is omitted or
None, the mapping is created without a name. Avoiding the use of the tag parameter will assist in keeping your code portable between Unix and Windows.
offset may be specified as a non-negative integer offset. mmap references will be relative to the offset from the beginning of the file. offset defaults to 0. offset must be a multiple of the ALLOCATIONGRANULARITY.
mmap(fileno, length, flags=MAP_SHARED, prot=PROT_WRITE|PROT_READ, access=ACCESS_DEFAULT[, offset])
(Unix version) Maps length bytes from the file specified by the file descriptor fileno, and returns a mmap object. If length is
0, the maximum length of the map will be the current size of the file when
flags specifies the nature of the mapping.
MAP_PRIVATEcreates a private copy-on-write mapping, so changes to the contents of the mmap object will be private to this process, and
MAP_SHAREDcreates a mapping that’s shared with all other processes mapping the same areas of the file. The default value is
prot, if specified, gives the desired memory protection; the two most useful values are
PROT_WRITE, to specify that the pages may be read or written. prot defaults to
PROT_READ | PROT_WRITE.
access may be specified in lieu of flags and prot as an optional keyword parameter. It is an error to specify both flags, prot and access. See the description of access above for information on how to use this parameter.
offset may be specified as a non-negative integer offset. mmap references will be relative to the offset from the beginning of the file. offset defaults to 0. offset must be a multiple of the PAGESIZE or ALLOCATIONGRANULARITY.
To ensure validity of the created memory mapping the file specified by the descriptor fileno is internally automatically synchronized with physical backing store on Mac OS X and OpenVMS.
This example shows a simple way of using
import mmap # write a simple example file with open("hello.txt", "wb") as f: f.write(b"Hello Python!\n") with open("hello.txt", "r+b") as f: # memory-map the file, size 0 means whole file mm = mmap.mmap(f.fileno(), 0) # read content via standard file methods print(mm.readline()) # prints b"Hello Python!\n" # read content via slice notation print(mm[:5]) # prints b"Hello" # update content using slice notation; # note that new content must have same size mm[6:] = b" world!\n" # ... and read again using standard file methods mm.seek(0) print(mm.readline()) # prints b"Hello world!\n" # close the map mm.close()
import mmap with mmap.mmap(-1, 13) as mm: mm.write(b"Hello world!")
New in version 3.2: Context manager support.
The next example demonstrates how to create an anonymous map and exchange data between the parent and child processes:
import mmap import os mm = mmap.mmap(-1, 13) mm.write(b"Hello world!") pid = os.fork() if pid == 0: # In a child process mm.seek(0) print(mm.readline()) mm.close()
Memory-mapped file objects support the following methods:
Closes the mmap. Subsequent calls to other methods of the object will result in a ValueError exception being raised. This will not close the open file.
Trueif the file is closed.
New in version 3.2.
find(sub[, start[, end]])¶
Returns the lowest index in the object where the subsequence sub is found, such that sub is contained in the range [start, end]. Optional arguments start and end are interpreted as in slice notation. Returns
Flushes changes made to the in-memory copy of a file back to disk. Without use of this call there is no guarantee that changes are written back before the object is destroyed. If offset and size are specified, only changes to the given range of bytes will be flushed to disk; otherwise, the whole extent of the mapping is flushed.
(Windows version) A nonzero value returned indicates success; zero indicates failure.
(Unix version) A zero value is returned to indicate success. An exception is raised when the call failed.
move(dest, src, count)¶
Copy the count bytes starting at offset src to the destination index dest. If the mmap was created with
ACCESS_READ, then calls to move will raise a
bytescontaining up to n bytes starting from the current file position. If the argument is omitted,
Noneor negative, return all bytes from the current file position to the end of the mapping. The file position is updated to point after the bytes that were returned.
Changed in version 3.3: Argument can be omitted or
Returns a byte at the current file position as an integer, and advances the file position by 1.
Returns a single line, starting at the current file position and up to the next newline.
Resizes the map and the underlying file, if any. If the mmap was created with
ACCESS_COPY, resizing the map will raise a
rfind(sub[, start[, end]])¶
Returns the highest index in the object where the subsequence sub is found, such that sub is contained in the range [start, end]. Optional arguments start and end are interpreted as in slice notation. Returns
Set the file’s current position. whence argument is optional and defaults to
0(absolute file positioning); other values are
1(seek relative to the current position) and
2(seek relative to the file’s end).
Return the length of the file, which can be larger than the size of the memory-mapped area.
Returns the current position of the file pointer.
Write the bytes in bytes into memory at the current position of the file pointer; the file position is updated to point after the bytes that were written. If the mmap was created with
ACCESS_READ, then writing to it will raise a