Python 在循环外获取变量

Question

I have a python code ,i need to get its value outside the for loop and if statements and use the variable further:

My code:

with open('text','r') as f:
  for line in f.readlines():
      if 'hi' in line
         a='hello'

print a  #variable requires outside the loop

But i get Nameerror: 'a' is not defined

Answer 1

The error message means you never assigned to a (ie the if condition never evaluated to True ).

To handle this more gracefully, you should assign a default value to a before the loop:

a = None
with open('test', 'r') as f:
   ...

You can then check if it's None after the loop:

if a is not None:
   ...

Answer 2

You may also try:

try:
    print a
except NameError:
    print 'Failed to set "a"'

EDIT: It simultaneously solves the problem of not printing a , if you did not find what you were looking for

Answer 3

The other answers here are correct—you need to guarantee that a has been assigned a value before you try to print it. However, none of the other answers mentioned Python's for ... else construct , which I think is exactly what you need here:

with open('text','r') as f:
  for line in f.readlines():
      if 'hi' in line:
          a='hello'
          break
  else:
      a='value not found'

print a  #variable requires outside the loop

The else clause is only run if the for loop finishes its last iteration without break -ing.

This construct seems unique to Python in my experience. It can be easily implemented in languages that support goto , but Python is the only language I know of with a built-in construct specifically for this. If you know of another such language, please leave a comment and enlighten me!

Answer 4

只需定义a=null或a=0成为全局变量，您可以在代码中的任何位置访问 a