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.

pressure = 103.9

def adjust(t):
    temperature = t * 1.43 / pressure
    return temperature

• pressure is a global variable.
  • Defined outside any particular function.
  • Visible everywhere.
• t and temperature are local variables in adjust.
  • 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('adjusted:', adjust(0.9))
print('temperature after call:', temperature)

adjusted: 0.01238691049085659

Traceback (most recent call last):
  File "/Users/swcarpentry/foo.py", line 8, in <module>
    print('temperature after call:', temperature)
NameError: name 'temperature' is not defined

Local and Global Variable Use

Trace the values of all variables in this program as it is executed. (Use ‘—’ as the value of variables before and after they exist.)

limit = 100

def clip(value):
    return min(max(0.0, value), limit)

value = -22.5
print(clip(value))

Solution

# limit = ---
# value = ---

limit = 100   

def clip(value):  
  return min(max(0.0, value), limit)

# limit = 100
# value = ---

value = -22.5    # value = -22.5, limit = 100

print(clip(value))   # result is 0.0

# value = -22.5
# limit = 100

Identifying Syntax Errors

1. Read the code below and try to identify what the errors are without running it.
2. Run the code and read the error message. Is it a SyntaxError or an IndentationError?
3. Fix the error.
4. Repeat steps 2 and 3 until you have fixed all the errors.

def another_function
  print("Syntax errors are annoying.")
   print("But at least Python tells us about them!")
  print("So they are usually not too hard to fix.")

Solution

There are missing parentheses and colon (): after the function call, and the print messages don’t appear aligned via whitespace

   File "<stdin>", line 1
    def another_function
                       ^
SyntaxError: invalid syntax

  File "<stdin>", line 1
    print("Syntax errors are annoying.")
    ^
IndentationError: unexpected indent  

  File "<stdin>", line 1
    print("But at least Python tells us about them!")
    ^
IndentationError: unexpected indent  

Working function:

def another_function():
  print("Syntax errors are annoying.")
  print("But at least Python tells us about them!")
  print("So they are usually not too hard to fix.")

Reading Error Messages

Read the traceback below, and identify the following:

1. How many levels does the traceback have?
2. What is the file name where the error occurred?
3. What is the function name where the error occurred?
4. On which line number in this function did the error occur?
5. What is the type of error?
6. What is the error message?

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-2-e4c4cbafeeb5> in <module>()
      1 import errors_02
----> 2 errors_02.print_friday_message()

/Users/ghopper/thesis/code/errors_02.py in print_friday_message()
     13
     14 def print_friday_message():
---> 15     print_message("Friday")

/Users/ghopper/thesis/code/errors_02.py in print_message(day)
      9         "sunday": "Aw, the weekend is almost over."
     10     }
---> 11     print(messages[day])
     12
     13

KeyError: 'Friday'

Solution

1. 3 levels, since there are 3 arrows
2. The file is errors_02.py
3. The function is print_message()
4. Line 11
5. It is a KeyError
6. There isn’t really a message; you’re supposed to infer that Friday is not a key in messages.

Key Points

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