CODEWITHSUNDEEP
pythonvariablesglobal
Kronixion
Kronixion

Reputation: 11

Can I use a global variable at the start of my code when I have defined it at the end of my program?

The code I am using is shown here:

import os.path

def def1():
    global filename
    if os.path.isfile(filename+ ".txt") == True:
        print ("Filename exists")
    if os.path.isfile(filename+ ".txt") == False:
        print("Filename dosent exist")

def def2():
    global filename
    filename = input("Please input the name if the file you want to see exists")

def Main():
    def1()
    def2()

Main()

What the code does is check whether or not a file exists, and the user can input the name of the file he/she wishes to check. This code will be used in a much bigger code file. I am not able to understand why I get this error when I run the code:

Traceback (most recent call last):
 File "/Users/Sid/Desktop/existfiletest.py", line 18, in <module>
  Main()
 File "/Users/Sid/Desktop/existfiletest.py", line 15, in Main
  def1()
 File "/Users/Sid/Desktop/existfiletest.py", line 5, in def1
  if os.path.isfile(filename+ ".txt") == True:
NameError: name 'filename' is not defined

I have defined the variable 'filename' as a global in def2(), but I start off the program by calling def1(). The variable 'filename' is a global, so I don't see why it can't be used in def1(). And also, if anyone says to call def2() first, yes that works. However I would like to know whether I can use a variable before defining it, as this is the case in my bigger code.

Thanks in advance.

Upvotes: 1

Views: 180

Answers (3)

wander95
wander95

Reputation: 1366

Sort answer:

replace:

    def Main():
       def1()
       def2()

by

    def Main():
       def2()
       def1()

Your mistake is not the fact that you defined the global variable earlier in the file. But by calling def1 first, you have left the variable undefined. So you must call def2 first to initialize and give it a value.

Secondly you do not need to perform the if test twice:

 def def1(filename):
     if os.path.isfile(filename+ ".txt") == True:
          print ("Filename exists")
     else:
          print("Filename does not exist")

Upvotes: 1

Kevin Schellenberg
Kevin Schellenberg

Reputation: 845

Save the filename in the main function and then pass it to the def1 function like so:

import os.path

def def1(filename):
    if os.path.isfile(filename+ ".txt") == True:
        print ("Filename exists")
    if os.path.isfile(filename+ ".txt") == False:
        print("Filename dosent exist")

def Main():
    filename = input("Please input the name if the file you want to see exists")
    def1(filename)

Main()

Upvotes: 0

Yevhen Kuzmovych
Yevhen Kuzmovych

Reputation: 12140

Don't use global variables. It is really bad practice. Use function arguments. You can change you code to:

import os.path

def def1(filename):
    if os.path.isfile(filename+ ".txt") == True:
        print ("Filename exists")
    if os.path.isfile(filename+ ".txt") == False:
        print("Filename dosent exist")

def def2():
    return input("Please input the name if the file you want to see exists")

def Main():
    filename = def2()
    def1(filename)

Main()

And of course you can't use variable before defining it. But you can define it with some default value.

Upvotes: 0

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