How to make global variables work in Spyder inside a function?

Question

I cannot modify global variables interactively in Spyder 4.0.0 with Python 3.8, Windows 10. There must have been a recent change because this was possible before.

I have the following example file:

x = 5
def IncreaseX():
    global x
    x += 1

print(x)
IncreaseX()
print(x)

1. When I run it (with F5), I get the desired output:

In [1]: runfile('TestGlobals.py', wdir='D:')
5
6

2. However, if I try to run the IncreaseX() function from the embedded Ipython console, it does not change the value of the global variable:

In [2]: x
Out[2]: 6

In [3]: IncreaseX()

In [4]: x
Out[4]: 6

3. The same occurs if I select the last 3 rows of my example, and I run them (with F9):

In [5]: print(x)
   ...: IncreaseX()
   ...: print(x)
6
6

4. The behaviour is different if I select all the rows of my example file and run them (with F9):

In [6]: x = 5
   ...: def IncreaseX():
   ...:     global x
   ...:     x += 1
   ...: 
   ...: print(x)
   ...: IncreaseX()
   ...: print(x)
5
6

5. After I do this, I can modify the value of the global variable. Repeating the exact same code of step 2., I obtain the desired outcome:

In [7]: x
Out[7]: 6

In [8]: IncreaseX()

In [9]: x
Out[9]: 7

Does anyone understand this behaviour? How can I restore the old behaviour (i.e., having step 2. to work directly as step 5.)?

I know that it is generally very bad to use global variables. However, I need to interactively modify some variables while I am controlling an experiment, hence a few correctly working global variables strongly simplify my workflow.

---

FOUND IT!!

Spyder/Preferences/Run/General settings/Run in console's namespace instead of an empty one

With this option turned on, the old behaviour of globals is restored.

Answer 1

Looks like you're confused with Python's (quite idiosyncratic) definition of what a "global" variable is -and you're not the only one actually, as it's rather different from the usual definition xD

In most languages, "global" means "process global" - a variable that is shared by all the modules / functions / whatever used by the current process.

Python does not have true "process global" scope and "global" actually means "module level" - each module is it's own namespace, and a global in a module is actually only "global" within this module. In the code using your module, you have to refer to this variable as "yourmodulename.x".

A short example may help...

mymodule.py:

x = 1

def inc():
   global x # within this module, this refers to `mymodule.x`, not any other 'x'
   x += 1

def show():
   # no need for the global kw if it's only for reading
   print("in mymodule, x = {}".format(x))

working.py:

import mymodule

print("mymodule.x before: {}".format(mymodule.x))
mymodule.show()
mymodule.inc()
print("mymodule.x after: {}".format(mymodule.x))
mymodule.show()

broken1.py:

import mymodule

# this 'x' belongs to broken1, and `mymodule` isn't even
# aware it exists... 
x = 42 
print("broken1.x before: {}".format(x))
mymodule.show() # setting broken1.x to 42 didn't change mymodule
mymodule.inc()
# our own 'x' has not changed
print("broken1.x after: {}".format(x))
# but mymodule.x did
mymodule.show()

Note that importing x from mymodule will not work as expected either - it only creates a new x in the importing module with the initial value of mymodule.x, but those names are STILL totally unrelated, and rebinding one will not rebind the other:

broken2.py:

from mymodule import x 
from mymodule import inc, show

print("broken2.x before: {}".format(x))
show() # so far we have the same values, BUT:
inc() # this rebinds mymodule.x but has no impact here
# our own 'x' has not changed
print("broken2.x after: {}".format(x))
# but mymodule.x did
show()

For more explanations on what Python's "variables" really are and why they behave that way, you definitly want to read Ned Batchelder's reference article here

So the solution to your problem is quite simply to always use mymodule.x (outside of mymodule of course):

working2.py:

import mymodule

mymodule.show()
mymodule.inc()
mymodule.show()

mymodule.x = 666
mymodule.show() # gives the expected result
# let's go to 667
mymodule.inc()
mymodule.show()