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# numpy.memmap

class `numpy.``memmap`

  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:
  > ‘r’ | Open existing file for reading only.
  > ‘r+’ | Open existing file for reading and writing.
  > ‘w+’ | Create or overwrite existing file for reading and writing.
  > ‘c’ | 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’.

  Notes

  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.

  Examples

      >>> 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)
      True

  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
      False

  Copy-on-write memmap:

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

  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)

  Attributes

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

  Methods

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