Global and Local Variables
阿新 • • 發佈：2019-02-19
http://www.python-course.eu/python3_global_vs_local_variables.php
In the following example, we want to demonstrate, how global values can be used inside the body of a function:
def f():
print(s)
s = "I love Paris in the summer!"
f()
The variable s is defined as the string "I love Paris in the summer!", before calling the function f(). The body of f() consists solely of
the "print(s)" statement. As there is no local variable s, i.e. no assignment to s, the value from the global variable s will be used. So the output will be the string "I love Paris in the summer!". The question is, what will happen, if we change the value
of s inside of the function f()? Will it affect the global variable as well? We test this in the following piece of code:def f():
s = "I love London!"
print(s)
s = "I love Paris!"
f()
print(s)
The output looks like this:
I love London! I love Paris!What if we combine the first example with the second one, i.e. first access s with a print() function, hoping to get the global value, and then assigning a new value to it? Assigning a value to it, would mean creating a local variable s. So, we would have s both as a global and a local variable in the same scope, i.e. the body of the function. Python doesn't allow this ambiguity. So, it will throw an error, as we can see in the following example:
>>> def f(): ... print(s) ... s = "I love London!" ... print(s) ... >>> s = "I love Paris!" >>> f() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in f UnboundLocalError: local variable 's' referenced before assignment >>>
def f():
global s
print(s)
s = "Only in spring, but London is great as well!"
print(s)
s = "I am looking for a course in Paris!"
f()
print(s)
We have solved our problem. There is no ambiguity left. The output of this small script looks like this:
I am looking for a course in Paris! Only in spring, but London is great as well! Only in spring, but London is great as well!Local variables of functions can't be accessed from outside, when the function call has finished:
def f():
s = "I am globally not known"
print(s)
f()
print(s)
Starting this script gives us the following output with an error message:
monty@python:~$ python3 ex.py
I am globally not known
Traceback (most recent call last):
File "ex.py", line 6, in <module>
print(s)
NameError: name 's' is not defined
monty@python:~$
The following example shows a wild combination of local and global variables and function parameters:
def foo(x, y):
global a
a = 42
x,y = y,x
b = 33
b = 17
c = 100
print(a,b,x,y)
a,b,x,y = 1,15,3,4
foo(17,4)
print(a,b,x,y)
The output looks like this:
42 17 4 17 42 15 3 4