os.getenv idiom

getenv_idiom.py

import os

def getenv(varname, default=None, typecast_fn=None):
    """
    Return the value of the environment variable varname if it exists, or default if it doesn't.
    default defaults to None.
    
    :typecast_fn: a function that performs the typecast. if not supplied, defaults to type(value)
    """
    
    if not typecast_fn:
        typecast_fn = type(default) if default else lambda x: x
        
    assert callable(typecast_fn), "typecast_fn is not callable!"
    
    value = os.getenv(varname)
    
    return typecast_fn(value) if value else default