lambda to assign a value to global variable?

Question

I'm using tkinter and trying to assign a value to a global variable on a button press. Here is the code:

popup.add_command(label="Allow Moving Item", command=lambda: allowMoving=True)

I'm getting the invalid syntax. Can you tell me how to work this around?

Avoid global variables. If you need an answer, refer to this http://stackoverflow.com/a/21054384/1627968. — Benjamin Rowell, Feb 13 '17 at 19:07
`command=lambda: allowMoving=True` --> `SyntaxError: can't assign to lambda` (`lambda`s may only contain an expression, no statements.) Replace it with the name of a normally defined function. — martineau, Feb 13 '17 at 19:21
Anonymous functions are defined by lambda *expressions*, which cannot contain an assignment (or any other kind of) *statement*. — chepner, Feb 13 '17 at 19:25

score 6 · Answer 1 · answered Feb 13 '17 at 19:35

For entertainment purposes only.

popup.add_command(label="Allow Moving Item",
                  command=lambda: globals().update(allowMoving=True))

(Although globals() is not documented with the same "do not modify the return value" warning as locals(), I'm still not sure this is guaranteed to work.)

A better answer would be to define the callback with a def statement instead.

def set_allow_moving():
    global allow_moving    # Don't use camel case for variable names in Python
    allow_moving = True

popup.add_command(label="Allow Moving Item", command=set_allow_moving)

Thanks, the globals().update(allowMoving=True) method works. The additional question - if it was local variable, can I change it without globals? P.S. I do not necessary need lambda — sssseossss, Feb 13 '17 at 22:17

score 5 · Answer 2 · answered Feb 14 '17 at 00:20

Don't use lambda. A good rule of thumb is to never use lambda unless there's simply no other way. The use of lambda in callbacks should be the exception rather than the rule.

def allow_moving():
    global allowMoving
    allowMoving = True

popup.add_command(label="Allow Moving Item", command=allow_moving)

lambda to assign a value to global variable?

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