This is an old copy of the Python FAQ. The information here may be outdated.

Can Python be compiled to machine code, C or some other language?

Not easily. Python’s high level data types, dynamic typing of objects and run-time invocation of the interpreter (using eval or exec) together mean that a “compiled” Python program would probably consist mostly of calls into the Python run-time system, even for seemingly simple operations like x+1.

Several projects described in the Python newsgroup or at past Python conferences have shown that this approach is feasible, although the speedups reached so far are only modest (e.g. 2x). Jython uses the same strategy for compiling to Java bytecode. (Jim Hugunin has demonstrated that in combination with whole-program analysis, speedups of 1000x are feasible for small demo programs. See the proceedings from the 1997 Python conference (dead link) for more information.)

Internally, Python source code is always translated into a bytecode representation, and this bytecode is then executed by the Python virtual machine. In order to avoid the overhead of repeatedly parsing and translating modules that rarely change, this byte code is written into a file whose name ends in “.pyc” whenever a module is parsed. When the corresponding .py file is changed, it is parsed and translated again and the .pyc file is rewritten.

There is no performance difference once the .pyc file has been loaded, as the bytecode read from the .pyc file is exactly the same as the bytecode created by direct translation. The only difference is that loading code from a .pyc file is faster than parsing and translating a .py file, so the presence of precompiled .pyc files improves the start-up time of Python scripts. If desired, the Lib/ module can be used to create valid .pyc files for a given set of modules.

Note that the main script executed by Python, even if its filename ends in .py, is not compiled to a .pyc file. It is compiled to bytecode, but the bytecode is not saved to a file. Usually main scripts are quite short, so this doesn’t cost much speed.

There are also several programs which make it easier to intermingle Python and C code in various ways to increase performance. See, for example, Psyco, Pyrex, PyInline, Py2Cmod, and Weave (dead link).


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