Python3.7
Python3 source file must be saved in UTF 8 encoding. 

Strings
Use single quote or double quote to quote string.

# single and double quotes are same in Python
a = "hello"
b = 'world'

print(a, b)  
You can use \n for linebreak, and \t for tab, etc.

Quoting Raw String 「r"…"」
You can add r in front of the quote symbol. This way, backslash characters will NOT be interpreted as escapes.

c = r"this\n and that"

Triple Quotes for Multi-Line String
To quote a string of multiple lines, use triple quotes.

d = """this
will be printed
in 3 lines"""

print(d)
substring, length
Substring extraction is done by appending a bracket str[begin_index:end_index]. 
Index can be negative, which counts from the end.


b="01234567"
print(b[1:4]) # prints “123”
Length of the string is len().

a="this"
print(len(a)) # 4
Strings can be joined by a plus sign +.

print("this" + " that")
String can be repeated using *.

print("this" * 2)
String Methods
Python: String Methods

Arithmetic

print(3 + 4) # 7
print(3 - 4) # -1
print(3 + - 4)   # -1
print(3 * 4) # 12

print(2 ** 3) # 8 power

print(11 / 5) # 2.2  (in python 2, this would be 2)
print(11 // 5)    # 2 (quotient)
print(11 % 5) # 1 remainder (modulo)

print(divmod(11, 5))  # (2, 1) quotient and remainder
Convert to {int, float, string}
Python doesn't automatically convert between {int, float, string}.

Convert to int: int(3.2)
Convert to float: float(3).
Convert to string, use “format” method. [see Python: Format String]
You can write with a dot after the number as float, like this: 3..
Assignment Operators

# add and assign
c = 0
c += 1
print(c)    # 1

# substract and assign
c = 0
c -= 2
print(c)    # -2

# multiply and assign
c = 2
c *= 3
print(c)    # 6

# exponent and assign
c = 3
c **= 2
print(c)    # 9

# divide and assign
c = 7
c /= 2
print(c)    # 3.5

# modulus (remainder) and assign
c = 13
c %= 5
print(c)    # 3

# quotient and assign
c = 13
c //= 5
print(c)    # 2
Note: Python doesn't support ++ or --.

Warning: ++i may not generate any error, but it doesn't do anything.


True and False
False like things, such as False, 0, empty string, empty array, …, all evaluate to False.

The following evaluate to False:

False. A builtin Boolean type.
None. A builtin type.
0. Zero.
0.0. Zero, float.
"". Empty string.
[]. Empty list.
(). Empty tuple.
{}. Empty dictionary.
set([]). Empty set.
frozenset([]). Empty frozen set.

my_thing = []

if my_thing:
    print("yes")
else:
    print("no")
Conditional: if then else

x = -1
if x<0:
    print('neg')
elif x==0:
    print('zero')
elif x==1:
    print('one')
else:
    print('other')

# the elif can be omitted.
Loop, Iteration
Example of a “for” loop.


a = list(range(1,5)) # creates a list from 1 to 4. (does NOT include the end)

for x in a:
    if x == 3:
        print(x)

# prints 3
The range(m, n) function gives a list from m to n-1.

Python also supports break and continue to exit loop.

break
Exit loop.
continue
Skip code and start the next iteration.


for x in range(1,9):
    print(x)
    if x == 4:
        break

# 1
# 2
# 3
# 4
Example of a “while” loop.


x = 1
while x <= 5:
    print(x)
    x += 1
List
Creating a list.

a = [0, 1, 2, "more", 4, 5, 6]
print(a)
Counting elements:

a = ["more", 4, 6]
print(len(a)) # prints 3
Getting a element. Use the syntax list[index]. Index start at 0. 
Negative index counts from right. Last element has index -1.

a = ["more", 4, 6]
print(a[1]) # prints 4
Extracting a sequence of elements (aka sublist, slice): list[start_index:end_index].

a = ["zero", "one", "two", "three", "four", "five", "six"]
print(a[2:4])   # prints ["two", "three"]
WARNING: The extraction is not inclusive. For example, mylist[2:4] returns only 2 elements, not 3.

Modify element: list[index] = new_value

xx = ["a", "b", "c"]
xx[2] = "two"
print(xx) # ['a', 'b', 'two']
A slice (continuous sequence) of elements can be changed by assigning to a list directly. 
The length of the slice need not match the length of new list.


xx = [ "b0", "b1", "b2", "b3", "b4", "b5", "b6"]

xx[0:6] = ["two", "three"]

print(xx)   # ['two', 'three', 'b6']
Nested Lists. Lists can be nested arbitrarily. Append extra bracket to get element of nested list.

a = [3, 4, [7, 8]]
print(a[2][1])    # returns 8

List Join. Lists can be joined with plus sign.

b = ["a", "b"] + [7, 6]
print(b)    # prints ['a', 'b', 7, 6]
Python: List Basics

Tuple
Python has a “tuple” type. It's like list, except it's immutable 
that is, the elements cannot be changed, nor added/deleted.

Syntax for tuble is using round brackets () instead of square brackets. 
The brackets are optional when not ambiguous, but best to always use them.

# tuple
t1 = (3, 4 , 5) # a tuple of 3 elements. paren optional when not ambiguous
print(t1) # (3, 4 , 5)
print(t1[0])    # 3

# nested tuple
t2 = ((3,8), (4,9), ("a", 5, 5))
print(t2[0])   # (3,8)
print(t2[0][0])    # 3

# a list of tuples
t3 = [(3,8), (4,9), (2,1)]
print(t3[0])   # (3,8)
print(t3[0][0])    # 3


Python Sequence Types
In Python, {string, list, tuple} are called “sequence types”. 
They all have the same methods. Here's example of operations that can be used on sequence type.


# operations on sequence types

# a list
ss = [0, 1, 2, 3]

# length
print(len(ss)) # 4

# ith item
print(ss[0]) # 0

# slice of items
print(ss[0:3])    # [0, 1, 2]

# slice of items with jump step
print(ss[0:10:2]) # [0, 2]

# check if a element exist
print(3 in ss)    # True. (or False)

# check if a element does NOT exist
print(3 not in ss) # False

# concatenation
print(ss + ss)   # [0, 1, 2, 3, 0, 1, 2, 3]

# repeat
print(ss * 2)    # [0, 1, 2, 3, 0, 1, 2, 3]

# smallest item
print(min(ss))    # 0

# largest item
print(max(ss))    # 3

# index of the first occurrence
print(ss.index(3))   # 3

# total number of occurrences
print(ss.count(3))   # 1


Dictionary: Key/Value Pairs
A keyed list in Python is called “dictionary” (known as Hash Table or Associative List in other languages). 
It is a unordered list of pairs, each pair is a key and a value.


# define a keyed list
aa = {"john":3, "mary":4, "joe":5, "vicky":7}

# getting value from a key
print(aa["mary"])
# 4

# add a entry
aa["pretty"] = 99

# delete a entry
del aa["vicky"]

print(aa)
# {'john': 3, 'mary': 4, 'joe': 5, 'pretty': 99}

# get keys
print(list(aa.keys()))
# ['john', 'mary', 'joe', 'pretty']

# get values
print(list(aa.values()))
# [3, 4, 5, 99]

# check if a key exists
print("is mary there:", "mary" in aa)
# is mary there: True


Loop Thru List/Dictionary
Here is a example of going thru a list by element.


myList = ['one', 'two', 'three', '∞']

for x in myList:
     print(x)
You can loop thru a list and get both {index, value} of a element.


myList = ['one', 'two', 'three']
for i, v in enumerate(myList):
     print(i, v)

# 0 one
# 1 two
# 2 three

Loop thru Dictionary
myDict = {"john":3, 'mary':4, 'joe':5, 'vicky':7}
for k, v in list(myDict.items()):
     print(k, v)

# output

# joe 5
# john 3
# mary 4
# vicky 7

Use Module
A library in Python is called a module.


# import the standard module named os
import os

# example of using a function
print('current dir is:', os.getcwd())
Python: List Modules, Search Path, Loaded Modules

Function
The following is a example of defining a function.

def myFun(x,y):
     """myFun returns x+y."""
     result = x+y
     return result

print(myFun(3,4)) # prints 7
Python: Function

Class and Object
Python: Class and Object

Writing a Module
Here's a basic example. Save the following line in a file and name it mymodule.py.


def f1(n):
    return n+1
To load the file, use import import module_name, then to call the function, use module_name.function_name.


import mymodule # import the module

print(mymodule.f1(5)) # calling its function. prints 6
print(mymodule.__name__)   # list its functions and variables