How to get a list of classes and functions from a python file without importing it

Question

I have a python file with some classes and functions defined in it:

class A(object):
    def __init__(self, an_arg, a_default_arg=None):
        pass

def doStuff(an_other_arg, an_other_default_arg=None):
    pass

And I want to get a list of all classes and functions in this file. (their names and parameter definitions are enough)

Now, I do know you can do this with __import__(module_descriptor) and inspect, but this is not an option as the file I'm scanning is from an untrusted source.

My first reaction was to try and create a safe environment to import them, but this seems impossible according to other stackoverflow-questions.

Answer 1

You can use the ast module to parse the source file, without actually executing any code. Then you can traverse the node tree to get the function and class names/parameters.

import ast

def show_info(functionNode):
    print("Function name:", functionNode.name)
    print("Args:")
    for arg in functionNode.args.args:
        #import pdb; pdb.set_trace()
        print("\tParameter name:", arg.arg)


filename = "untrusted.py"
with open(filename) as file:
    node = ast.parse(file.read())

functions = [n for n in node.body if isinstance(n, ast.FunctionDef)]
classes = [n for n in node.body if isinstance(n, ast.ClassDef)]

for function in functions:
    show_info(function)

for class_ in classes:
    print("Class name:", class_.name)
    methods = [n for n in class_.body if isinstance(n, ast.FunctionDef)]
    for method in methods:
        show_info(method)

Result:

Function name: doStuff
Args:
        Parameter name: an_other_arg
        Parameter name: an_other_default_arg
Class name: A
Function name: __init__
Args:
        Parameter name: self
        Parameter name: an_arg
        Parameter name: a_default_arg

Answer 2

The accepted solution is incomplete. Consider the following file:

def regular_function():
    def nested_function():
        pass

async def async_function():
    pass

The accepted solution will only print:

Function name: regular_function
Args:

To get all functions, we need to make two changes:

1. Walk the entire AST, rather than just top level nodes
2. Handle async functions as well as regular functions

Here is the corrected code, for finding functions:

import ast

from pathlib import Path

parsed_ast = ast.parse(Path(__file__).read_text())

functions = [
    node
    for node in ast.walk(parsed_ast)
    if isinstance(node, (ast.FunctionDef, ast.AsyncFunctionDef))
]

for function in functions:
    print(f"Function name: {function.name}")
    print(f"Args: {', '.join([arg.arg for arg in function.args.args])}")

Note that this is pushing up against the bounds of what an AST walk should be used for. For anything more complicated, consider using NodeVisitor