Pythonic way of using type as function argument

Question

What would be the most pythonic way to pass an object type as an agrgument in a function?

Let me give you an example. Let's say I was trying to get a configuration from an environment variable. Because all environment variables are strings I need to cast the value to the correct type.

To do this I need to tell the function the desired type. This is the purpose of the coerce argument. My first instinct is to pass in the desired type as the value for coerce. However, I am not sure if there are any implications or problems in doings so.

import os

# The Function
def get_config(config: str, coerce: type, default: any, delimiter: str = ","):
    value = os.getenv(config, None)  # Get config from environment
    if value is None:
        return default  # Return default if config is None

    if coerce is bool:
        value = str2bool(value)  # Cast config to bool
    elif coerce is int:
        value = str2int(value)  # Cast config to int
    elif coerce is list:
        value = value.split(delimiter)  # Split string into list on delimiter

    return value  # Return the config value

# Usage
os.environ["TEST_VAR"] = "True"

test_var = get_config("TEST_VAR", bool, False)

print(test_var)  #  output is True
print(type(test_var))  # output is <class 'bool'>

To me this seems more clear and pythonic than using a string such as "str" or "bool" to specify the type. However, I would like to know if there could be any problems caused by passing around built in types as function arguments.

Answer 1

You can simplify the code and make it more powerful by just directly passing the conversion function (type annotations are left as an exercise):

def get_config(config, convert, default):
    value = os.getenv(config, None)
    return default if value is None else convert(value)

test_var = get_config("TEST_VAR", str2bool, False)

and perhaps having a helper function for the list case:

def make_str2list(delimiter=','):
    return lambda s: s.split(delimiter)

test_var = get_config("TEST_VAR", make_str2list(':'), [])

Answer 2

Since all you are doing with the type argument is to compare it one by one to certain specific types that you're expecting, rather than actually using type to construct objects of that type, it is doing nothing different from passing in a string such as 'str' or 'bool' as an argument and compare it to several string constants.

Instead, you can make the conversion functions such as str2bool and str2int themselves an argument, so that you can call coerce(value) to convert value in a generic way. Store such conversion functions as attributes of a dedicated class for better readability, as demonstrated below:

import os
import typing

class to_type:
    bool = 'True'.__eq__
    int = int
    list = lambda s: s.split(',')

def get_config(config: str, coerce: typing.Callable = lambda s: s, default: any = None):
    value = os.getenv(config, None)
    if value is None:
        return default
    return coerce(value)

os.environ["TEST_VAR"] = "True"
print(get_config("TEST_VAR", to_type.bool))
os.environ["TEST_VAR"] = "2"
print(get_config("TEST_VAR", to_type.int))
os.environ["TEST_VAR"] = "a,b,c"
print(get_config("TEST_VAR", to_type.list))
os.environ["TEST_VAR"] = "foobar"
print(get_config("TEST_VAR"))

This outputs:

True
2
['a', 'b', 'c']
foobar