Guide to using filemagic

Background

libmagic is the library that commonly supports the file command on Unix system, other than Max OSX which has its own implementation. The library handles the loading of database files that describe the magic numbers used to identify various file types, as well as the associated mime types. The library also handles character set detections.

Installation

Before installing filemagic, the libmagic library will need to be availabile. To test this is the check for the presence of the file command and/or the libmagic man page.

$ which file
$ man libmagic

On Mac OSX, Apple has implemented their own version of the file command. However, libmagic can be installed using homebrew

$ brew install libmagic

After brew finished installing, the test for the libmagic man page should pass.

Now that the presence of libmagic has been confirmed, use pip to install filemagic.

$ pip install filemagic

The magic module should now be availabe from the Python shell.

>>> import magic

The next section will describe how to use the magic.Magic class to identify file types.

Usage

The magic module uses ctypes to wrap the primitives from libmagic in the more user friendly magic.Magic class. This class handles initialization, loading databases and the release of resources.

>>> import magic

To ensure that resources are correctly released by magic.Magic, it’s necessary to either explicitly call close() on instances, or use with statement.

>>> with magic.Magic() as m:
...     pass
...

magic.Magic supports context managers which ensures resources are correctly released at the end of the with statements irrespective of any exceptions.

To identify a file from it’s filename, use the id_filename() method.

>>> with magic.Magic() as m:
...     m.id_filename('setup.py')
...
'Python script, ASCII text executable'

Similarily to identify a file from a string that has already been read, use the id_buffer() method.

>>> with magic.Magic() as m:
...     m.id_buffer('#!/usr/bin/python\n')
...
'Python script, ASCII text executable'

To identify with mime type, rather than a textual description, pass the MAGIC_MIME_TYPE flag when creating the magic.Magic instance.

>>> with magic.Magic(flags=magic.MAGIC_MIME_TYPE) as m:
...     m.id_filename('setup.py')
...
'text/x-python'

Similarily, MAGIC_MIME_ENCODING can be passed to return the encoding type.

>>> with magic.Magic(flags=magic.MAGIC_MIME_ENCODING) as m:
...     m.id_filename('setup.py')
...
'us-ascii'

Memory management

The libmagic library allocates memory for its own use outside that Python. This memory needs to be released when a magic.Magic instance is no longer needed. The preferred way to doing this is to explicitly call the close() method or use the with statement, as described above.

Starting with version 1.4 magic.Magic this memory will be automatically cleaned up when the instance is garbage collected. However, unlike CPython, some Python interpreters such as PyPy, Jython and IronPython do not have deterministic garbage collection. Because of this, filemagic will issue a warning if it automatically cleans up resources.

Unicode and filemagic

On both Python2 and Python3, magic.Magic‘s methods will encode any unicode objects (the default string type for Python3) to byte strings before being passed to libmagic. On Python3, returned strings will be decoded to unicode using the default encoding type. The user should not be concerned whether unicode or bytes are passed to magic.Magic methods. However, the user will need to be aware that returned strings are always unicode on Python3 and byte strings on Python2.

Reporting issues

The source code for filemagic is hosted on Github. Problems can be reported using Github’s issues tracking system.

filemagic has been tested against libmagic 5.11. Continuous integration is provided by Travis CI. The current build status is .