Why are .pyc files created on import?

Python source code is compiled to bytecode, and it is the bytecode that is run. A .pyc file contains a copy of that bytecode, and by caching that Python doesn't have to re-compile the Python code each time it needs to load the module.

You can get an idea of how much time is saved by timing the compile() function:

>>> import urllib2>>> import timeit>>> urllib2_source = open(urllib2.__file__.rstrip('c')).read()>>> timeit.timeit("compile(source, '', 'exec')", 'from __main__ import urllib2_source as source', number=1000)6.977046966552734>>> _ / 1000.00.006977046966552734

So it takes 7 milliseconds to compile the urllib2.py source code. That doesn't sound like much, but this adds up quickly as Python loads a lot of modules in its lifetime. Just run an average script with the -v command-line switch; here I run the help output for the pydoc tool:

$ bin/python -v -m pydoc -h# installing zipimport hookimport zipimport # builtin# installed zipimport hook# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/site.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/site.pyimport site # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/site.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/os.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/os.pyimport os # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/os.pycimport errno # builtinimport posix # builtin# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/posixpath.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/posixpath.pyimport posixpath # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/posixpath.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/stat.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/stat.pyimport stat # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/stat.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/genericpath.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/genericpath.pyimport genericpath # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/genericpath.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/warnings.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/warnings.pyimport warnings # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/warnings.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/linecache.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/linecache.pyimport linecache # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/linecache.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/types.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/types.pyimport types # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/types.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/UserDict.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/UserDict.pyimport UserDict # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/UserDict.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/_abcoll.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/_abcoll.pyimport _abcoll # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/_abcoll.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/abc.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/abc.pyimport abc # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/abc.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/_weakrefset.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/_weakrefset.pyimport _weakrefset # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/_weakrefset.pycimport _weakref # builtin# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/copy_reg.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/copy_reg.pyimport copy_reg # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/copy_reg.pycimport encodings # directory /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/__init__.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/__init__.pyimport encodings # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/__init__.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/codecs.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/codecs.pyimport codecs # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/codecs.pycimport _codecs # builtin# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/aliases.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/aliases.pyimport encodings.aliases # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/aliases.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/utf_8.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/utf_8.pyimport encodings.utf_8 # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/encodings/utf_8.pycPython 2.7.8 (default, Sep  9 2014, 11:33:29) [GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwinType "help", "copyright", "credits" or "license" for more information.# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/runpy.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/runpy.pyimport runpy # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/runpy.pycimport imp # builtin# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/pkgutil.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/pkgutil.pyimport pkgutil # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/pkgutil.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/re.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/re.pyimport re # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/re.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_compile.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_compile.pyimport sre_compile # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_compile.pycimport _sre # builtin# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_parse.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_parse.pyimport sre_parse # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_parse.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_constants.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_constants.pyimport sre_constants # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/sre_constants.pycdlopen("/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/_locale.so", 2);import _locale # dynamically loaded from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/_locale.so# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/inspect.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/inspect.pyimport inspect # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/inspect.pyc# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/string.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/string.pyimport string # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/string.pycdlopen("/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/strop.so", 2);import strop # dynamically loaded from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/strop.so# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/dis.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/dis.pyimport dis # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/dis.pyc# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/opcode.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/opcode.pyimport opcode # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/opcode.pyc# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/tokenize.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/tokenize.pyimport tokenize # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/tokenize.pycdlopen("/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/itertools.so", 2);import itertools # dynamically loaded from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/itertools.so# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/token.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/token.pyimport token # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/token.pycdlopen("/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/operator.so", 2);import operator # dynamically loaded from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/operator.so# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/collections.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/collections.pyimport collections # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/collections.pycdlopen("/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/_collections.so", 2);import _collections # dynamically loaded from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/_collections.so# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/keyword.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/keyword.pyimport keyword # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/keyword.pyc# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/heapq.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/heapq.pyimport heapq # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/heapq.pycdlopen("/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/_heapq.so", 2);import _heapq # dynamically loaded from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/_heapq.soimport thread # builtin# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/repr.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/repr.pyimport repr # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/repr.pyc# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/traceback.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/traceback.pyimport traceback # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/traceback.pyc# /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/locale.pyc matches /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/locale.pyimport locale # precompiled from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/locale.pyc# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/functools.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/functools.pyimport functools # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/functools.pycdlopen("/Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/_functools.so", 2);import _functools # dynamically loaded from /Users/mpieters/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/_functools.so# /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/getopt.pyc matches /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/getopt.pyimport getopt # precompiled from /Users/mpietre/Development/Library/buildout.python/parts/opt/lib/python2.7/getopt.pycpydoc - the Python documentation toolpydoc.py <name> ...    Show text documentation on something.  <name> may be the name of a    Python keyword, topic, function, module, or package, or a dotted    reference to a class or function within a module or module in a    package.  If <name> contains a '/', it is used as the path to a    Python source file to document. If name is 'keywords', 'topics',    or 'modules', a listing of these things is displayed.pydoc.py -k <keyword>    Search for a keyword in the synopsis lines of all available modules.pydoc.py -p <port>    Start an HTTP server on the given port on the local machine.pydoc.py -g    Pop up a graphical interface for finding and serving documentation.pydoc.py -w <name> ...    Write out the HTML documentation for a module to a file in the current    directory.  If <name> contains a '/', it is treated as a filename; if    it names a directory, documentation is written for all the contents.# clear __builtin__._# clear sys.path# clear sys.argv# clear sys.ps1# clear sys.ps2# clear sys.exitfunc# clear sys.exc_type# clear sys.exc_value# clear sys.exc_traceback# clear sys.last_type# clear sys.last_value# clear sys.last_traceback# clear sys.path_hooks# clear sys.path_importer_cache# clear sys.meta_path# clear sys.flags# clear sys.float_info# restore sys.stdin# restore sys.stdout# restore sys.stderr# cleanup __main__# cleanup[1] _collections# cleanup[1] locale# cleanup[1] functools# cleanup[1] encodings# cleanup[1] site# cleanup[1] runpy# cleanup[1] operator# cleanup[1] supervisor# cleanup[1] _heapq# cleanup[1] abc# cleanup[1] _weakrefset# cleanup[1] sre_constants# cleanup[1] collections# cleanup[1] _codecs# cleanup[1] opcode# cleanup[1] _warnings# cleanup[1] mpl_toolkits# cleanup[1] inspect# cleanup[1] encodings.utf_8# cleanup[1] repr# cleanup[1] codecs# cleanup[1] getopt# cleanup[1] pkgutil# cleanup[1] _functools# cleanup[1] thread# cleanup[1] keyword# cleanup[1] strop# cleanup[1] signal# cleanup[1] traceback# cleanup[1] itertools# cleanup[1] posix# cleanup[1] encodings.aliases# cleanup[1] exceptions# cleanup[1] _weakref# cleanup[1] token# cleanup[1] dis# cleanup[1] tokenize# cleanup[1] heapq# cleanup[1] string# cleanup[1] imp# cleanup[1] zipimport# cleanup[1] re# cleanup[1] _locale# cleanup[1] sre_compile# cleanup[1] _sre# cleanup[1] sre_parse# cleanup[2] copy_reg# cleanup[2] posixpath# cleanup[2] errno# cleanup[2] _abcoll# cleanup[2] types# cleanup[2] genericpath# cleanup[2] stat# cleanup[2] warnings# cleanup[2] UserDict# cleanup[2] os.path# cleanup[2] linecache# cleanup[2] os# cleanup sys# cleanup __builtin__# cleanup ints: 21 unfreed ints# cleanup floats

That's 53 imports:

$ bin/python -v -m pydoc -h 2>&1 | egrep ^import | wc -l      53

rather than load the (larger) source file each time and compiling it, a smaller bytecode file can be read and used immediately. That easily adds up to 1/3rd or 1/2 a second just to print some help information for a command-line tool.

Python does not create a cache file for the main script; that's because that would clutter up your scripts directory with files that are not going to be loaded nearly as often as modules are loaded.

If you run a script that often that the compile time for that one file affects you, you can always either move the majority of the code to a module (and avoid having to compile a large script) or you can use the compileall tool to create a .pyc cache file for the script, then run that .pyc file directly. Note that Python then will not recompile that file if you changed the script!