Python: __pycache__ directory

To improve the performance of loading python modules, python caches the compiled version of every module that you use.

For example

arithmetic.py

def sum(a, b):
  return a+b

def subtract(a, b):
  return a-b
  
def mul(a,b):
  return a*b
  
def div(a, b):
  return a/b

main.py

import arithmetic

if __name__ == "__main__":
    import sys
    a = int(sys.argv[1])
    b = int(sys.argv[2])
    
    print("a = ", a, "b = ", b)
    print(arithmetic.sum(a,b))
    print(arithmetic.subtract(a,b))
    print(arithmetic.mul(a,b))
    print(arithmetic.div(a,b))

$ ls
arithmetic.py main.py
$ 
$ python3 main.py 10 20
a =  10 b =  20
30
-10
200
0.5
$ 
$ ls
__pycache__ arithmetic.py main.py
$
$ cd __pycache__/
$ ls
arithmetic.cpython-35.pyc

Observe above output, python creates __pycache__ directory and caches the compiled version of arithmetic.py file. The compiled version of python files follows the format 'module.version.pyc'. In my case, I am using python 3.5, so the file name is arithmetic.cpython-35.pyc. This naming convention allows compiled modules from different releases and different versions of Python to coexist. Major advantage of these compiled modules is that, these are platform independent, so you can ship these to other platforms and use as it is.

Note

A program doesn’t run any faster when it is read from a .pyc file than when it is read from a .py file; the only thing that’s faster about .pyc files is the speed with which they are loaded.

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