What does “global variables are bad” mean?

Question

So I can read from a global variable

def f() :
  print x

And I can also assign it

def g()
  global x
  x = 3

Answer 1

Global variables can be changed and access from nearly everywhere. This includes maybe colliding namespace and assignments,where you don't want to have them.

To give an example:

def someFunction(): 
    global myVar 
    myVar = "fuuu" 
    print myVar

myVar = "baa" 
f() 
print myVar 

Will end in something like.

fuuu
fuuu 

Whereas you might expected

fuuu 
baa

So I'd say both variants are little dangerous.