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Python Library Reference

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7.7mmap --Memory-mapped file support

Memory-mapped file objects behave like both strings and likefile objects. Unlike normal string objects, however, these aremutable. You can use mmap objects in most places where stringsare expected; for example, you can use there module tosearch through a memory-mapped file. Since they're mutable, you canchange a single character by doingobj[index] = 'a', orchange a substring by assigning to a slice:obj[i1:i2] = '...'. You can also read and writedata starting at the current file position, andseek()through the file to different positions.

A memory-mapped file is created by themmap() function,which is different on Unix and on Windows. In either case you mustprovide a file descriptor for a file opened for update.If you wish to map an existing Python file object, use itsfileno() method to obtain the correct value for thefileno parameter. Otherwise, you can open the file using theos.open() function, which returns a file descriptordirectly (the file still needs to be closed when done).

For both the Unix and Windows versions of the function,access may be specified as an optional keyword parameter.access accepts one of three values:ACCESS_READ,ACCESS_WRITE, orACCESS_COPY to specifyreadonly, write-through or copy-on-write memory respectively.access can be used on both Unix and Windows. Ifaccess is not specified, Windows mmap returns a write-throughmapping. The initial memory values for all three access types aretaken from the specified file. Assignment to anACCESS_READ memory map raises aTypeErrorexception. Assignment to anACCESS_WRITE memory mapaffects both memory and the underlying file. Assigment to anACCESS_COPY memory map affects memory but does not updatethe underlying file.

mmap(fileno, length[, tagname[, access]])

(Windows version) Mapslength bytes from the file specified by the file handlefileno, and returns a mmap object. Iflength is0, the maximum length of the map will be the current size of the file whenmmap() is called.

tagname, if specified and notNone, 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 orNone, 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.

mmap(fileno, length[, flags[, prot[, access]]])

(Unix version) Mapslength bytes from the file specified by the file descriptorfileno, and returns a mmap object.

flags specifies the nature of the mapping.MAP_PRIVATE creates a private copy-on-write mapping, so changes to the contents of the mmap object will be private to this process, andMAP_SHARED creates a mapping that's shared with all other processes mapping the same areas of the file. The default value isMAP_SHARED.

prot, if specified, gives the desired memory protection; the two most useful values arePROT_READ andPROT_WRITE, to specify that the pages may be read or written.prot defaults toPROT_READ | PROT_WRITE.

access may be specified in lieu offlags andprot as an optional keyword parameter. It is an error to specify bothflags,prot andaccess. See the description ofaccess above for information on how to use this parameter.

Memory-mapped file objects support the following methods:

close()

Close the file. Subsequent calls to other methods of the object will result in an exception being raised.

find(string[, start])

Returns the lowest index in the object where the substringstring is found. Returns-1 on failure.start is the index at which the search begins, and defaults to zero.

flush([offset, size])

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. Ifoffset andsize are specified, only changes to the given range of bytes will be flushed to disk; otherwise, the whole extent of the mapping is flushed.

move(dest,src,count)

Copy thecount bytes starting at offsetsrc to the destination indexdest. If the mmap was created withACCESS_READ, then calls to move will throw aTypeError exception.

read(num)

Return a string containing up tonum bytes starting from the current file position; the file position is updated to point after the bytes that were returned.

read_byte()

Returns a string of length 1 containing the character at the current file position, and advances the file position by 1.

readline()

Returns a single line, starting at the current file position and up to the next newline.

resize(newsize)

If the mmap was created withACCESS_READ orACCESS_COPY, resizing the map will throw aTypeError exception.

seek(pos[, whence])

Set the file's current position.whence argument is optional and defaults to0 (absolute file positioning); other values are1 (seek relative to the current position) and2 (seek relative to the file's end).

size()

Return the length of the file, which can be larger than the size of the memory-mapped area.

tell()

Returns the current position of the file pointer.

write(string)

Write the bytes instring 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 withACCESS_READ, then writing to it will throw aTypeError exception.

write_byte(byte)

Write the single-character stringbyte into memory at the current position of the file pointer; the file position is advanced by1.If the mmap was created withACCESS_READ, then writing to it will throw aTypeError exception.

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Python Library Reference

Previous:7.6.1 Queue ObjectsUp:7. Optional Operating SystemNext:7.8 anydbm

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Release 2.2.3, documentation updated on 30 May 2003.