Preprocessing function text in runtime bofore compilation

Question

I decided to try to preprocess function text before it\'s compilation into byte-code and following execution. This is merely for training. I hardly imagine situations where it\'

Answer 1

One can avoid creating a temporary file by invoking the exec statement on the source. (You can also explicitly call compile prior to exec if you want additional control over compilation, but exec will do the compilation for you, so it's not necessary.) Correctly calling exec has the additional benefit that the function will work correctly if it accesses global variables from the namespace of its module.

The problem described in the second question can be resolved by temporarily blocking the decorator while it is running. That way the decorator remains, along all the other ones, but is a no-op.

Here is the updated source.

from __future__ import print_function

import sys


def a():
    print('a()')


def comment_1(s):
    lines = s.split('\n')
    return '\n'.join(line.replace(';','#;',1) if line.strip().startswith('1;') else line for line in lines)

_blocked = False

def remove_1(f):
    global _blocked
    if _blocked:
        return f
    import inspect
    source = inspect.getsource(f)    
    new_source = comment_1(source)
    env = sys.modules[f.__module__].__dict__
    _blocked = True
    try:
        exec new_source in env
    finally:
        _blocked = False
    return env[f.__name__]


@remove_1
def f():
    1;a()
    print('Some statements 1')
    1;a()
    print('Some statements 2')


f()

def remove_1(f):    
    import inspect
    source = inspect.getsource(f)    
    new_source = comment_1(source)
    env = sys.modules[f.__module__].__dict__.copy()
    exec new_source in env
    return env[f.__name__]

Answer 2

I'll leave a modified version of the solution given in the answer by user4815162342. It uses ast module to delete some parts of f, as was suggested in the comment to the question. To make it I majorly relied on the information in this article.

This implementation deletes all occurrences of a as standalone expression.

from __future__ import print_function
import sys
import ast
import inspect


def a():
    print('a() is called')


_blocked = False

def remove_1(f):
    global _blocked
    if _blocked:
        return f
    import inspect
    source = inspect.getsource(f)

    a = ast.parse(source) #get ast tree of f

    class Transformer(ast.NodeTransformer):
        '''Will delete all expressions containing 'a' functions at the top level'''
        def visit_Expr(self, node): #visit all expressions
            try:
                if node.value.func.id == 'a': #if expression consists of function with name a
                    return None #delete it
            except(ValueError):
                pass
            return node #return node unchanged
    transformer = Transformer()
    a_new = transformer.visit(a)
    f_new_compiled = compile(a_new,'<string>','exec')

    env = sys.modules[f.__module__].__dict__
    _blocked = True
    try:
        exec(f_new_compiled,env)
    finally:
        _blocked = False
    return env[f.__name__]


@remove_1
def f():
    a();a()
    print('Some statements 1')
    a()
    print('Some statements 2')


f()

The output is:

Some statements 1
Some statements 2