Variable Scope

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How do function calls actually work?

• How can I determine where errors occurred?

Objectives

• Identify local and global variables.

• Identify parameters as local variables.

• Read a traceback and determine the file, function, and line number on which the error occurred, the type of error, and the error message.

The scope of a variable is the part of a program that can ‘see’ that variable.

• There are only so many sensible names for variables.
• People using functions shouldn’t have to worry about what variable names the author of the function used.
• People writing functions shouldn’t have to worry about what variable names the function’s caller uses.
• The part of a program in which a variable is visible is called its scope.

access_url = 'http://memegenerator.net/'

def extract_domain(uri):
    domain = uri[7:-1]
    return domain

• access_url is a global variable.
  • Defined outside any particular function.
  • Visible everywhere.
• uri and domain are local variables in extract_domain.
  • Defined in the function.
  • Not visible in the main program.
  • Remember: a function parameter is a variable that is automatically assigned a value when the function is called.

print('domain is:', extract_domain('http://metafilter.com/'))
print('url after call:', domain)

domain is: metafilter.com

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-22-9860e4547781> in <module>()
      1 print('domain is:', extract_domain('http://metafilter.com/'))
----> 2 print('url after call:', domain)
      3 

NameError: name 'domain' is not defined

Key Points

• The scope of a variable is the part of a program that can ‘see’ that variable.