Python: Functions & Importing

[last updated: 2021-08-08]
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• Define a Function:
  • Code to define a function starts with "def"
    followed by the function name,
    followed by "( )" that contain any parameters passed to the function,
    followed by a colon ":"
  • The statements that make up the body of the function follow, and must be indented from the "def" line
  • A "return" statement or an un-indented line will mark the end of a function definition
    Note: a return statement is good practice,
    since if eg. you have a null function with nothing in it other than perhaps a comment,
    it will fail with error if there's no return statement.
    (same applies - according to my trials - if you only have file open & close statements in the function)

  • For example:
    def calcFunc ():
    tempVar = 5
    return
• Using a Function:
• Using a function inside the program where it's defined:
  • call a function with:
    calcFunc()
  • The function call must be executed After the function is defined in your program.
• Using a function in a different program (in the same directory):
  (Note: These procedures worked in both python 2.7.15 and python 3.5.3)
  Functions must be "imported" to use them in a different program:
  Suppose you have defined a function, eg. func01
  and it's defined (located) in eg. funcs.py
  and you want to use this function in a different program, eg. main.py
  • Put these lines into main.py:
    • import funcs
      then execute with:
      funcs.func01()
    • or:
      import funcs as F1 (where 'F1' is your choice of name/tag)
      then execute with:
      F1.func01()
    • or:
      from funcs import func01 (only imports the func01 function from funcs)
      then execute with:
      func01()
    one source recommended using: from .funcs import func01
    but this failed with error something about "relative imports" and "parent not loaded"
    • or:
      from funcs import * (imports all functions defined in funcs)
      then execute the same, with:
      func01()
    However: one note said this method failed with python 3.5.4...
• Using a function in a different program (in a different directory):
• (3.5.3) if funcs.py is located in a sub-directory myFuncs, then this worked:
  from myFuncs.funcs import *
  However I could not find an analogous syntax for directories above current directory...
• ...
• [stuff below here not tested...]
• or:
  import sys
  print(sys.path) # shows your current sys path
  sys.path.append(" **Put here the directory where you have the file with your function** ")
  then execute with:
  from file import function
• or:
  Add the path to the PYTHONPATH variable:
  export PYTHONPATH=$HOME/dirWithScripts/:$PYTHONPATH
• or:
  import os
  os.chdir(" **Put here the directory where you have the file with your function** ")
  then execute with:
  from file import function
  then restore working directory:
  os.chdir(" **Put here the directory where you were working** ")
• or:
  # some_file.py
  import sys
  # insert at 1, 0 is the script path (or '' in REPL)
  sys.path.insert(1, '/path/to/application/app/folder')
  import file
• or:
  from application.app.folder.file import func_name
  Note: Just make sure folder also contains an __init__.py, this allows it to be included as a package.
  The init.py or __init__.py file can be an empty file. Its very existence tells Python to treat the directory as a package.
• or (in summary?):
  First: import sys
  Second append the folder path: sys.path.insert(0, '/the/folder/path/name-folder/')
  Third Make a blank file called __ init __.py in your subdirectory (this tells Python it is a module):
  name-file.py
  name-folder
  __ init __.py
  name-module.py

  Fourth import the module inside the folder: from name-folder import name-module

• or:
  import sys
  sys.path.append(pathToFolderContainingScripts)
  from scriptName import functionName #scriptName without .py extension
• ...

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• Misc Notes to Edit:
  • If sub.py is just a collection of functions that you've defined,
    then you can import them and call them from main.py with this:
    import sub Note: Do NOT include the .py of your sub.py
    sub.func_to_call()
    Confirmed
  • If sub.py also contains statements,
    then they will be executed (once) when you do the import

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