Geography 465 Modules and Functions Writing Custom Functions

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Geography 465 Modules and FunctionsWriting
Custom Functions
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Python Modules

• Each Python script is a module
• Modules can access other modules
• Modules can use variables and/or constants
• Modules can use functions

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Writing a Function

• What makes a good function
• Single, clear objective
• Reusable by intent
• Function syntax
• Def defines the function
• Return() sends result back to calling module
• See example

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Syntax Example

• MyCustomFunction.py
• Def ltnamegt (arg1, arg2, argN)
• your custom function code
• return ltvaluegt

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Code example result of two numbers

• MyFunctions.py
• def calculate(num1, num2, mathOp)
• result eval(str(num1) mathOp str(num2))
• return result

Interactive Window gtgtgt import MyFunctions gtgtgt
MyFunctions.calculate (3, 4, ) gtgtgt 7
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Using variables in functions

• Assignment statement
• Variables at module level are available to
  calling script
• Variables within a function are local to the
  function

Script1.py import NewYork nyLat
NewYork.lat nyLong NewYork.long nyCalc
NewYork.calcgrowth (7000000, 0.03) Print nyCalc
Passing values to module function
NewYork.py lat -73.905 Global long
40.708 Global def calcgrowth(pop,
rate) growth pop rate return growth
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Importing a custom module

• First time in Python
• Finds the module
• Compiles the module
• Runs the module
• Subsequent times (same Python session)
• Re-uses imported (compiled) module

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Finding the module

• Where does Python look to find module?
• The search path hierarchy is
• Home directory of calling module
• PYTHONPATH environment variable
• Standard Python library directions
• Contents of .pth (text) files

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Adding a custom search path

• Modify sys.path
• Append location of your custom modules
• Last for duration of script
• Resets to default when PythonWin closes

Example.py import sys sys.path.append(C\\MyModu
les) Print sys.path
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Reloading a modulesingle Python session

• Module load only once
• Script1.py NewYork.py
• import NewYork Access NewYork.py lat -73
• long 40

• To re-import, call reload()
• Interactive Window NewYork.py
• gtgtgt reload(NewYork) Access modified lat
  -73.905
• NewYork.py long 40.708

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Running the module

• All statements execute from top to bottom
• Statements do not exist until Python reaches and
  runs code
• Code inside functions do not run until called

NewYork.py lat -73.905 long 40.708 def
calcgrowth (pop, rate) growth pop
rate return growth
Follow indent rules