Functions

Built-in Functions

• dir(__builtins__ )
• common useful functions
  • dir(), help(0
  • id()
  • exists()
    • object, variable exist or not
  • range(),len()
  • open()
  • max(sequence, [key]), min()

User Defined Functions

• def( *args, **kargs)
  • key word parameters
  • support "named parameters(kargs)
  • *stands for an arbitrary number,* also pack variables as a tuple (can use * to pack)
  • return multiple values by returning a tuple
  • type hints and type aliases (Vector = List[float])

• def greeting(name: str) -> str:
      return 'Hello ' + name
  greeting('Charles')
  

• function has a name, can compare by environmental variable
  • if __name\_ = funcname:()_
• when the main function calls the function, must be after the definition
  • when other functions calls it, need not to be defined before

function can be nested (define function within another function)

• inside function is encapsulated, not accessible in global scope

• closure - function returns another function, often used together with decorator

  • def nth_power(exponent):
        def exponent_of(base):
            return base ** exponent
        return exponent_of 
    
    square = nth_power(2) 
    cube = nth_power(3)  
    square
    # output
    <function __main__.nth_power.<locals>.exponent(base)>
    
    cube
    # output
    <function __main__.nth_power.<locals>.exponent(base)>
    
    print(square(2)) 
    print(cube(2)) 
    # output
    4 # 2^2
    8 # 2^3
    

call by neither value, nor reference - passed by assigment/object reference

• variables are assigned by "binding name to values", when a=b, a,b are bind into same value
• name (reference) in stack, object in heap
• when change a to a = val, a is bind to a different value and id(storage place)
• when pass as an argument
  • re-assign will not change the object (create a new object instead)
  • modify will affect the object

function usually has no side effects ( change only return value)

• in functional programming (all passed by value, not mutable when re-asign, modify)
• function can have side effects by changing (eg. += ) mutable types (eg. list) in function
• immutable type cannot be changed

variable is dynamically typed

• python 3 has type hint

Variable Scope

• global key word
  • indicates this is a varaible out of local scope
  • cannot modify global variables value inside a local function
• nonlocal keyword
  • indicates this is a variable from outside scope in nested functions

Lambda

unamed functions

• lambda <parameters>: <expression>
  • lambda is an expression not a statement

Input/output

• input(<prompt>) function
  • raw_input(<prompt>) in Python2
  • is a string, use int(), float() to cast
• print variable holder in {}:
  • print("hello, {}", format(s) ) or print("hello, {s}")
  • Use format string (see string)
  • line change : add argument end = " ", no change: end = ' '
• get command-line input/output :
  • sys library: sys.argc, sys.argv

Module, Package, Library

• A complete file is a module
  • from module import ... ( * can be a wildcard -everything )
  • import module [as ...]
  • module functions can be assigned to variables
• A hierarchical file directory structure is a package
  • defines the execution environment for Python applications (consisting of submodules and sub-packages)
  • Library is a collection of modules with related functions