How to change a string in a function when calling a function?

Question

I'm not sure if this is possible but is there a way to change a string that a function prints while calling it from another function? I want to do something like this:

def string():
    print ("This cat was scared.")

def main():
    for words in string():
        str.replace("cat", "dog")
        # Print "The do was scared."

main()

Answer 1

By popular demand (well, one person's curiosity…), here's how you actually could change the string in a function before calling that function.

You should never do this in practice. There are some use cases for playing around with code objects, but this really isn't one of them. Plus, if you do anything less trivial, you should use a library like bytecode or byteplay instead of doing it manually. Also, it goes without saying that not all Python implementations use CPython-style code objects. But anyway, here goes:

import types

def string():
    print ("This cat was scared.")

def main():
    # A function object is a wrapper around a code object, with
    # a bit of extra stuff like default values and closure cells.
    # See inspect module docs for more details.
    co = string.__code__
    # A code object is a wrapper around a string of bytecode, with a
    # whole bunch of extra stuff, including a list of constants used
    # by that bytecode. Again see inspect module docs. Anyway, inside
    # the bytecode for string (which you can read by typing
    # dis.dis(string) in your REPL), there's going to be an
    # instruction like LOAD_CONST 1 to load the string literal onto
    # the stack to pass to the print function, and that works by just
    # reading co.co_consts[1]. So, that's what we want to change.
    consts = tuple(c.replace("cat", "dog") if isinstance(c, str) else c
                   for c in co.co_consts)
    # Unfortunately, code objects are immutable, so we have to create
    # a new one, copying over everything except for co_consts, which
    # we'll replace. And the initializer has a zillion parameters.
    # Try help(types.CodeType) at the REPL to see the whole list.
    co = types.CodeType(
        co.co_argcount, co.co_kwonlyargcount, co.co_nlocals,
        co.co_stacksize, co.co_flags, co.co_code,
        consts, co.co_names, co.co_varnames, co.co_filename,
        co.co_name, co.co_firstlineno, co.co_lnotab,
        co.co_freevars, co.co_cellvars)
    string.__code__ = co
    string()

main()

---

If that's not hacky enough for you: I mentioned that code objects are immutable. And of course so are strings. But deep enough under the covers, they're just pointer to some C data, right? Again, only if we're using CPython, but if we are…

First, grab my superhackyinternals project off GitHub. (It's intentionally not pip-installable because you really shouldn't be using this except to experiment with your local build of the interpreter and the like.) Then:

import ctypes
import internals

def string():
    print ("This cat was scared.")

def main():
    for c in string.__code__.co_consts:
        if isinstance(c, str):
            idx = c.find('cat')
            if idx != -1:
                # Too much to explain here; see superhackyinternals
                # and of course the C API docs and C source.
                p = internals.PyUnicodeObject.from_address(id(c))
                assert p.compact and p.ascii
                length = p.length
                addr = id(c) + internals.PyUnicodeObject.utf8_length.offset
                buf = (ctypes.c_int8 * 3).from_address(addr + idx)
                buf[:3] = b'dog'

    string()

main()

Answer 2

As a guess:

• You wanted string() to return a value the caller can use, instead of just printing something to the screen. So you need a return statement instead of a print call.
• You want to loop over all of the words in that returned string, not all the characters, so you need to call split() on the string.
• You want to replace stuff in each word, not in the literal "cat". So, you need to call replace on word, not on the str class. Also, replace doesn't actually change the word, it returns a new one, which you have to remember.
• You want to print out each of those words.

If so:

def string():
    return "This cat was scared."

def main():
    for word in string().split():
        word = word.replace("cat", "dog")
        print(word, end=' ')
    print()

main()

That fixes all of your problems. However, it can be simplified, because you don't really need word.replace here. You're swapping out the entire word, so you can just do this:

def main():
    for word in string().split():
        if word == "cat": word = "dog"
        print(word, end=' ')
    print()

But, even more simply, you can just call replace on the entire string, and you don't need a loop at all:

def main():
    print(string().replace("cat", "dog"))

Answer 3

What I think you may actually be looking for, is the ability to call your function with a default argument:

def string(animal='cat'):
    print("This {} was scared.".format(animal))

>>> string()
This cat was scared.

>>> string('dog')
This dog was scared.

If you pass nothing to string, the default value is assumed. Otherwise, the string prints with the substring you explicitly passed.