how to make a python function as built-in function or the alike?

Question

I want to test whether modules exists in python or not? but there seems no direct solution, so I wrote a function as below:

vim

I hope everytime I open the python interactive interface, I can simply type

test_module(module_name) 

and thus check whether a module is existent or not.

so how can I make this function as something like built-in function so as to reach my target?

thanks!

• Kevin: I have an alternative proposal for checking whether a module exists. Type import myModule. If it says ImportError, then it doesn't exist. You can also see a list of all modules with help("modules")

Answer 1

You can add it to the __builtin__ module:

import __builtin__
def test_module(module_name):
 # do something here
__builtin__.test_module = test_module

In Python 3, the module is called builtins instead.

If you want this to be run every time you open your Python interpreter, you can create a usercustomize.py module in USER_SITE location; it'll be imported everytime you run Python.

Be careful with expanding the built-ins, however. Adding names there makes them accessible globally by all Python code, and if any such code has accidentally or deliberately uses test_module where a NameError should have been raised, now uses your custom function.

It's much better to put such things into a dedicated module and only import this when you actually need that function. Explicit is better than implicit.

• user3401853: you meant if I use builtin module, the function can't be imported everytime I run python? then what is the usage of builtin module?
• Martijn Pieters: No, I mean that if you add the function to __builtin__ it may mask errors.
• user3401853: I don't understand. what is the relation between my question and "builtin may mask errors"?
• Martijn Pieters: @user3401853: Say you are running code that explicitly tests if foobar is set, catches the NameError and handles that by setting something. The code expects foobar not to exist. Then you add foobar to the Python built-ins, you now broke that module.
• Martijn Pieters: @user3401853: Adding a new name as a built-in affects all Python code, and you cannot always predict how that code is going to handle the new name in the namespace.