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How to define global function in Python?

stackoverflow.com › questions › 27930038 › how-to-define-global-function-in-python

Functions are added to the current namespace like any other name would be added. That means you can use the global keyword inside a function or method:

Copydef create_global_function():
    global foo
    def foo(): return 'bar'

The same applies to a class body or method:

Copyclass ClassWithGlobalFunction:
    global spam
    def spam(): return 'eggs'

    def method(self):
        global monty
        def monty(): return 'python'

with the difference that spam will be defined immediately as top-level class bodies are executed on import.

Like all uses of global you probably want to rethink the problem and find another way to solve it. You could return the function so created instead, for example.

Demo:

Copy>>> def create_global_function():
...     global foo
...     def foo(): return 'bar'
... 
>>> foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined
>>> create_global_function()
>>> foo
<function foo at 0x102a0c7d0>
>>> foo()
'bar'
>>> class ClassWithGlobalFunction:
...     global spam
...     def spam(): return 'eggs'
...     def method(self):
...         global monty
...         def monty(): return 'python'
... 
>>> spam
<function spam at 0x102a0cb18>
>>> spam()
'eggs'
>>> monty
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'monty' is not defined
>>> ClassWithGlobalFunction().method()
>>> monty()
'python'

Answer from Martijn Pieters on Stack Overflow

Quora

quora.com › How-can-you-create-or-use-a-global-variable-in-a-function-in-Python

How to create or use a global variable in a function in Python - Quora

Answer (1 of 11): With care. There is rarely a time when globals are neccessary (in any language - not just in Python). As a beginner (which I assume you are) they are a short cut, but they are a bad habit to get into. As you progress, you will realize that using global have the following effect...

Stack Overflow

stackoverflow.com › questions › 27930038 › how-to-define-global-function-in-python

How to define global function in Python? - Stack Overflow

Top answer

1 of 4

46

Functions are added to the current namespace like any other name would be added. That means you can use the global keyword inside a function or method:

Copydef create_global_function():
    global foo
    def foo(): return 'bar'

The same applies to a class body or method:

Copyclass ClassWithGlobalFunction:
    global spam
    def spam(): return 'eggs'

    def method(self):
        global monty
        def monty(): return 'python'

with the difference that spam will be defined immediately as top-level class bodies are executed on import.

Like all uses of global you probably want to rethink the problem and find another way to solve it. You could return the function so created instead, for example.

Demo:

Copy>>> def create_global_function():
...     global foo
...     def foo(): return 'bar'
... 
>>> foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined
>>> create_global_function()
>>> foo
<function foo at 0x102a0c7d0>
>>> foo()
'bar'
>>> class ClassWithGlobalFunction:
...     global spam
...     def spam(): return 'eggs'
...     def method(self):
...         global monty
...         def monty(): return 'python'
... 
>>> spam
<function spam at 0x102a0cb18>
>>> spam()
'eggs'
>>> monty
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'monty' is not defined
>>> ClassWithGlobalFunction().method()
>>> monty()
'python'

2 of 4

12

You can use global to declare a global function from within a class. The problem with doing that is you can not use it with a class scope so might as well declare it outside the class.

Copyclass X:
  global d
  def d():
    print 'I might be defined in a class, but I\'m global'

>> X.d

   Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   AttributeError: 'X' object has no attribute 'd'

>> d()

I might be defined in a class, but I'm global

Discussions

Why would one define "global variables" in python functions? How to use these correctly?

Imma just quote Code Complete, page 338, "Reasons to Use Global Data" Used with discipline, global variables are useful in several situations: Preservation of global values Sometimes you have data that applies conceptually to your whole program. This might be a variable that reflects the state of a program—for example, interactive vs. command-line mode, or normal vs. error-recovery mode. Or it might be information that’s needed throughout a program—for example, a data table that every routine in the program uses. Emulation of named constants Although C++, Java, Visual Basic, and most modern languages support named constants, some languages such as Python, Perl, Awk, and UNIX shell script still don’t. You can use global variables as substitutes for named constants when your language doesn’t support them. For example, you can replace the literal values 1 and 0 with the global variables TRUE and FALSE set to 1 and 0, or you can replace 66 as the number of lines per page with LINES_PER_PAGE = 66. It’s easier to change code later when this approach is used, and the code tends to be easier to read. This disciplined use of global data is a prime example of the distinction between programming in vs. programming into a language, which is discussed more in Section 34.4, “Program into Your Language, Not in It.” Emulation of enumerated types You can also use global variables to emulate enumerated types in languages such as Python that don’t support enumerated types directly. Streamlining use of extremely common data Sometimes you have so many references to a variable that it appears in the parameter list of every routine you write. Rather than including it in every parameter list, you can make it a global variable. However, in cases in which a variable seems to be accessed everywhere, it rarely is. Usually it’s accessed by a limited set of routines you can package into a class with the data they work on. More on this later. Eliminating tramp data Sometimes you pass data to a routine or class merely so that it can be passed to another routine or class. For example, you might have an error-processing object that’s used in each routine. When the routine in the middle of the call chain doesn’t use the object, the object is called “tramp data.” Use of global variables can eliminate tramp data. That said, most of the chapter is dedicated to why you should NOT use global data and how to avoid doing so... Most experienced programmers have concluded that using global data is riskier than using local data. Most experienced programmers have also concluded that access to data from several routines is pretty useful. Even if global variables don’t always produce errors, however, they’re hardly ever the best way to program. More on reddit.com

r/learnpython

23

December 31, 2017

How to make variables global by default, Python 3.11

So if I want to use them in a function I have to use the global prefix/keyword. # In main block. myvar = '' # Here's my function. def myfunc(): global myvar myvar = "hi" return myvar print(myfunc()) in Python 3.11 is there a way to make all these variables global by default. I ... More on discuss.python.org

discuss.python.org

0

June 10, 2024

Why can't imported functions access global variables?

“Global” is something of a misnomer; symbols defined in top-level scope are actually only in module scope. There actually aren’t global variables in Python the way you might expect that to mean. Ultimately the key is to write functions that don’t depend on global state. More on reddit.com

r/learnpython

6

4

December 24, 2018

Question about AWS Lambda, python, and globals

The code that sets the global variable will be evaluated once per Execution Context when your module is imported. After that, the value of the variable is in memory and there's no need to calculate it again.

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