Variable Scope

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.

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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.

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print('adjusted:', adjust(0.9))
print('temperature after call:', temperature)

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adjusted: 0.01238691049085659

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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

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limit = 100

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

value = -22.5
print(clip(value))

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---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-2-e4c4cbafeeb5in <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'