Remove type hints in Python source programmatically

Question

I have some source code written for Python 3.5 that I want to make executable under Python 3.4. The only feature from 3.5 that I use which is not available in 3.4 are type h

Answer 1

You can subclass lib2to3.refactor.RefactoringTool to refactor the code using a fixer that is a subclass of lib2to3.fixer_base.BaseFix with a pattern that looks for either a typed argument, a function declaration with an annotated returning value, or a simple statement that imports or imports from typing, and a transform method that removes the indices of the annotations from the child nodes or replaces the statement node with an empty node:

from lib2to3 import fixer_base, refactor, fixer_util

class FixParameterAnnotations(fixer_base.BaseFix):
    PATTERN = r'''
        name=tname
        |
        func=funcdef< any+ '->' any+ >
        |
        simple_stmt<
            (
                import_name< 'import' 'typing' >
                |
                import_from< 'from' 'typing' 'import' any+ >
            ) '\n'
        >
    '''

    def transform(self, node, results):
        if 'name' in results:
            del node.children[1:] # delete annotation to typed argument
        elif 'func' in results:
            del node.children[-4:-2] # delete annotation to function declaration
        else:
            return fixer_util.BlankLine() # delete statement that imports typing
        return node

class Refactor(refactor.RefactoringTool):
    def __init__(self, fixers):
        self._fixers= [cls(None, None) for cls in fixers]
        super().__init__(None, {'print_function': True})

    def get_fixers(self):
        return self._fixers, []

so that:

source = """
import typing
from typing import Dict, T, Callable
from typing import List

def foo(bar: Dict[T, List[T]],
        baz: Callable[[T], int] = lambda x: (x+3)/7,
        **kwargs) -> List[T]:
    print(line, end="")    # comments and white spaces are preserved
"""
print(Refactor([FixParameterAnnotations]).refactor_string(source, ''))

outputs:

def foo(bar,
        baz = lambda x: (x+3)/7,
        **kwargs):
    print(line, end="")    # comments and white spaces are preserved

Demo: https://repl.it/@blhsing/BurlywoodFeistyTrials

As a bonus, lib2to3 also preserves all comments and white spaces after the transformation. You can find the definition of the Python grammar in Grammar.txt of the lib2to3 module.