!!!Python
-----------------------

(P2) indicates Python 2 \\
(P3) indicates Python 3

!Running

Create a file named "file1.py"

{{{python3  file1.py}}}



!Data Types - all are objects

| NoneType	| None (like null or nil)
| bool		| False / True (also acts like 0 / 1)
| int		| unlimited size (P3), 23 (0x22 hex,  0o34 octal, 0b010 binary)
| long		| P2 only,  48L
| float		| 3.141
| str		| “string”  or  ‘string’ - immutable
|		| characters are just strings of length 1
| [[a,b]		| list - mutable
| (a,b)		| tuple - immutable

!Data Conversion

| int(x)	|	convert to int
| long(x)	|	P2 only
| str(x)	|	convert to str

!Misc

Comments start with #

All variables are object references (boxed)

Identifiers start with a letter or underscore - case sensitive

All variables are dynamically typed

{{type(x)}} returns the class of {{x}}

Zero based indexing using [[].  Also allows negative indexing from the back (-1 = last character)

!Logical operations

In conditionals, the following are all false:
|	False
|	None
|	0
|	“”
All else is true.

| x is y	|	do x and y refer to the same object
| x is not y  |
| ==		| compares values, works on strings
| 1 < x > 10 |	range check
| 2 in [[1, 2, 3, 4] |
| ‘el’ in ‘hello there’ |


Logical operators (work like lisp!)
|and
|or
|not

!Function Definition

{{{def fun():
	line 1
	line 2}}}

Can use {{{return x}}} (without {{{return}}}, returns None)

Function names that begin with underscore are private (by convention - not enforced)

Statement grouping is indicated by space characters at the beginning of each line rather than braces, {{{begin}}} / {{{end}}} statements, etc.

All variables are local unless declared with {{global}}


!Statements

{{{if cond1:
	stmt 1
elif cond2:
	stmt 2
else:
	stmt 3}}}


{{{while cond:
	stmts}}}


{{{for var in lst:
	stmt using var}}}

{{{try:
	stmts
except type as var:
	stmts}}}


!Operators

{{{+=}}}   	works on all data types

Use {{{"""string"""}}} to embed newlines in string

{{{pass}}} is statement that does nothing (used where statements are required)


!Modules & Packages

(Application) Module X = X.py file

Also system modules like:  sys  os  etc.

{{{import  file1}}}        # imports into the file "file1.py"
{{{import  file1, file2, file3
import  file1 as mymodule}}}

or

{{{from file1 import *}}}        # imports into the current package

{{{from file1 import (obj1, obj2, …)}}}

Accessing objects imported via the lines starting with “import” is done:
   {{{ file1.obj1}}}
Accessing objects imported via the lines starting with “from” is done:
    {{{obj1}}}

Python searches for modules as follows:

# same directory as file with the import statement
# PYTHONPAH environment variable
# sys.path

import only loads it the first time but will not re-load a changed file

Reloading a changed file can be done with {{{reload(file1)}}}

!Input / Output


| print(x)	|			prints x and newline (P3)
| x = input(“prompt”)	|	input string with prompt (P3)
| x = raw_input(“prompt”) |	input string with prompt (P2)
| x = input()		|	input string without prompt (throws EOFError exception)


!Classes

Top of hierarchy is “object”

Supports multiple-inheritance & metaclasses

{{{class MyClass:
	...}}}

{{{class MyClass (SuperClass1, SuperClass2, …):
         ...}}}

{{{class MyClass:
    cv1 = None
    def __init__(self):
        self.iv1 = None
        self.iv2 = None
    def getiv1(self):
        return self.iv1
    def setiv1(self, val):
    	self.iv1 = val
    def getcv1():
        return MyClass.cv1
    def setcv1(val):
        MyClass.cv1 = val
    def __repr__(self):   #  printed representation (also str() if no
        return "aabb"     #  __str__
    def __str__(self):   #  str() representation
        return "xxxxyyyy"   

x = MyClass()
MyClass.setcv1(55)
x.setiv1(33)

MyClass.cv1 = 77
x.iv1 = 32

class MyClass2(MyClass):
    def __init__(self):
        super().__init__()}}}
        
        

!Dictionary

{{{dict = {}
dict[‘key’] = value
dict.clear()
len(dict)
dict.get(‘key’)
dict.has_key(‘key’)
dict.keys()}}}

!Virtual environments

{{{mkdir myProject				create virtual environment
cd myProject
virtualenv venv

source venv/bin/activate		enter virtual environment

deactivate				exit virtual environment}}}