tiny os.getenv idiom

2017-01-15

Many times I add the possibility to control constant values via environment variables, and add a default value when no such variable is set. for instance:

import os  
A_CONSTANT_NUMBER = int(os.getenv(CONSTANT_NUMBER, 10))

I find myself writing this code again and again. IMO, a more idiomic solution would be:

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

Now, you can write the following:

A_NUMBER = getenv(A_NUMBER)

the above code will behave just like os.getenv.

A_NUMBER = getenv(A_NUMBER, typecast_fn=int)

Now the code will lookup the environment variable A_NUMBER . if successful, will cast it to int and return it. otherwise, will return None (just like os.getenv):

A_NUMBER = getenv(A_NUMBER, 10)

the above code will lookup the environment variable A_NUMBER.
if successful, will cast it to int and return it. otherwise, will return the default.