Is it possible to modify variable in python that i

Question

Given following code:

def A() :
    b = 1

    def B() :
        # I can access 'b' from here.
        print( b )
        # But can i modify 'b' here? 'global' and assignment will not work.

    B()
A()

For the code in B() function variable b is in outer scope, but not in global scope. Is it possible to modify b variable from within B() function? Surely I can read it from here and print(), but how to modify it?

Answer 1

I'm a little new to Python, but I've read a bit about this. I believe the best you're going to get is similar to the Java work-around, which is to wrap your outer variable in a list.

def A():
   b = [1]
   def B():
      b[0] = 2
   B()
   print(b[0])

# The output is '2'

Edit: I guess this was probably true before Python 3. Looks like nonlocal is your answer.

Answer 2

I don't think you should want to do this. Functions that can alter things in their enclosing context are dangerous, as that context may be written without the knowledge of the function.

You could make it explicit, either by making B a public method and C a private method in a class (the best way probably); or by using a mutable type such as a list and passing it explicitly to C:

def A():
    x = [0]
    def B(var): 
        var[0] = 1
    B(x)
    print x

A()

Answer 3

No you cannot, at least in this way.

Because the "set operation" will create a new name in the current scope, which covers the outer one.

Answer 4

Python 3.x has the nonlocal keyword. I think this does what you want, but I'm not sure if you are running python 2 or 3.

The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope. This is important because the default behavior for binding is to search the local namespace first. The statement allows encapsulated code to rebind variables outside of the local scope besides the global (module) scope.

For python 2, I usually just use a mutable object (like a list, or dict), and mutate the value instead of reassign.

example:

def foo():
    a = []
    def bar():
        a.append(1)
    bar()
    bar()
    print a

foo()

Outputs:

[1, 1]

Answer 5

You can use an empty class to hold a temporary scope. It's like the mutable but a bit prettier.

def outer_fn():
   class FnScope:
     b = 5
     c = 6
   def inner_fn():
      FnScope.b += 1
      FnScope.c += FnScope.b
   inner_fn()
   inner_fn()
   inner_fn()

This yields the following interactive output:

>>> outer_fn()
8 27
>>> fs = FnScope()
NameError: name 'FnScope' is not defined

Answer 6

The short answer that will just work automagically

I created a python library for solving this specific problem. It is released under the unlisence so use it however you wish. You can install it with pip install seapie or check out the home page here https://github.com/hirsimaki-markus/SEAPIE

user@pc:home$ pip install seapie

from seapie import Seapie as seapie
def A():
    b = 1

    def B():
        seapie(1, "b=2")
        print(b)

    B()
A()

outputs

2

the arguments have following meaning:

• The first argument is execution scope. 0 would mean local B(), 1 means parent A() and 2 would mean grandparent <module> aka global
• The second argument is a string or code object you want to execute in the given scope
• You can also call it without arguments for interactive shell inside your program

---

The long answer

This is more complicated. Seapie works by editing the frames in call stack using CPython api. CPython is the de facto standard so most people don't have to worry about it.

The magic words you are probably most likely interesed in if you are reading this are the following:

frame = sys._getframe(1)          # 1 stands for previous frame
parent_locals = frame.f_locals    # true dictionary of parent locals
parent_globals = frame.f_globals  # true dictionary of parent globals

exec(codeblock, parent_globals, parent_locals)

ctypes.pythonapi.PyFrame_LocalsToFast(ctypes.py_object(frame),ctypes.c_int(1))
# the magic value 1 stands for ability to introduce new variables. 0 for update-only

The latter will force updates to pass into local scope. local scopes are however optimized differently than global scope so intoducing new objects has some problems when you try to call them directly if they are not initialized in any way. I will copy few ways to circumvent these problems from the github page

• Assingn, import and define your objects beforehand
• Assingn placeholder to your objects beforehand
• Reassign object to itself in main program to update symbol table: x = locals()["x"]
• Use exec() in main program instead of directly calling to avoid optimization. Instead of calling x do: exec("x")

If you are feeling that using exec() is not something you want to go with you can emulate the behaviour by updating the the true local dictionary (not the one returned by locals()). I will copy an example from https://faster-cpython.readthedocs.io/mutable.html

import sys
import ctypes

def hack():
    # Get the frame object of the caller
    frame = sys._getframe(1)
    frame.f_locals['x'] = "hack!"
    # Force an update of locals array from locals dict
    ctypes.pythonapi.PyFrame_LocalsToFast(ctypes.py_object(frame),
                                          ctypes.c_int(0))

def func():
    x = 1
    hack()
    print(x)

func()

Output:

hack!