class numpy.memmap[source]

Create a memory-map to an array stored in a binary file on disk.

Memory-mapped files are used for accessing small segments of large files on disk, without reading the entire file into memory. Numpy’s memmap’s are array-like objects. This differs from Python’s mmap module, which uses file-like objects.

This subclass of ndarray has some unpleasant interactions with some operations, because it doesn’t quite fit properly as a subclass. An alternative to using this subclass is to create the mmap object yourself, then create an ndarray with ndarray.__new__ directly, passing the object created in its ‘buffer=’ parameter.

This class may at some point be turned into a factory function which returns a view into an mmap buffer.

Parameters :

filename : str or file-like object

The file name or file object to be used as the array data buffer.

dtype : data-type, optional

The data-type used to interpret the file contents. Default is uint8.

mode : {‘r+’, ‘r’, ‘w+’, ‘c’}, optional

The file is opened in this mode:


Open existing file for reading only.


Open existing file for reading and writing.


Create or overwrite existing file for reading and writing.


Copy-on-write: assignments affect data in memory, but changes are not saved to disk. The file on disk is read-only.

Default is ‘r+’.

offset : int, optional

In the file, array data starts at this offset. Since offset is measured in bytes, it should normally be a multiple of the byte-size of dtype. When mode != 'r', even positive offsets beyond end of file are valid; The file will be extended to accommodate the additional data. The default offset is 0.

shape : tuple, optional

The desired shape of the array. If mode == 'r' and the number of remaining bytes after offset is not a multiple of the byte-size of dtype, you must specify shape. By default, the returned array will be 1-D with the number of elements determined by file size and data-type.

order : {‘C’, ‘F’}, optional

Specify the order of the ndarray memory layout: C (row-major) or Fortran (column-major). This only has an effect if the shape is greater than 1-D. The default order is ‘C’.


The memmap object can be used anywhere an ndarray is accepted. Given a memmap fp, isinstance(fp, numpy.ndarray) returns True.

Memory-mapped arrays use the Python memory-map object which (prior to Python 2.5) does not allow files to be larger than a certain size depending on the platform. This size is always < 2GB even on 64-bit systems.


>>> data = np.arange(12, dtype='float32')
>>> data.resize((3,4))

This example uses a temporary file so that doctest doesn’t write files to your directory. You would use a ‘normal’ filename.

>>> from tempfile import mkdtemp
>>> import os.path as path
>>> filename = path.join(mkdtemp(), 'newfile.dat')

Create a memmap with dtype and shape that matches our data:

>>> fp = np.memmap(filename, dtype='float32', mode='w+', shape=(3,4))
>>> fp
memmap([[ 0.,  0.,  0.,  0.],
        [ 0.,  0.,  0.,  0.],
        [ 0.,  0.,  0.,  0.]], dtype=float32)

Write data to memmap array:

>>> fp[:] = data[:]
>>> fp
memmap([[  0.,   1.,   2.,   3.],
        [  4.,   5.,   6.,   7.],
        [  8.,   9.,  10.,  11.]], dtype=float32)
>>> fp.filename == path.abspath(filename)

Deletion flushes memory changes to disk before removing the object:

>>> del fp

Load the memmap and verify data was stored:

>>> newfp = np.memmap(filename, dtype='float32', mode='r', shape=(3,4))
>>> newfp
memmap([[  0.,   1.,   2.,   3.],
        [  4.,   5.,   6.,   7.],
        [  8.,   9.,  10.,  11.]], dtype=float32)

Read-only memmap:

>>> fpr = np.memmap(filename, dtype='float32', mode='r', shape=(3,4))
>>> fpr.flags.writeable

Copy-on-write memmap:

>>> fpc = np.memmap(filename, dtype='float32', mode='c', shape=(3,4))
>>> fpc.flags.writeable

It’s possible to assign to copy-on-write array, but values are only written into the memory copy of the array, and not written to disk:

>>> fpc
memmap([[  0.,   1.,   2.,   3.],
        [  4.,   5.,   6.,   7.],
        [  8.,   9.,  10.,  11.]], dtype=float32)
>>> fpc[0,:] = 0
>>> fpc
memmap([[  0.,   0.,   0.,   0.],
        [  4.,   5.,   6.,   7.],
        [  8.,   9.,  10.,  11.]], dtype=float32)

File on disk is unchanged:

>>> fpr
memmap([[  0.,   1.,   2.,   3.],
        [  4.,   5.,   6.,   7.],
        [  8.,   9.,  10.,  11.]], dtype=float32)

Offset into a memmap:

>>> fpo = np.memmap(filename, dtype='float32', mode='r', offset=16)
>>> fpo
memmap([  4.,   5.,   6.,   7.,   8.,   9.,  10.,  11.], dtype=float32)


filename str Path to the mapped file.
offset int Offset position in the file.
mode str File mode.


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