Expression

Expression is a sequence of operator s and operand s

>>> 1 + 1
2
>>> a = [1,2,3]
>>> 1 + 1 + 1 + 1
4
>>> 1 + 2 * 3
7
>>> 1 * 2 + 3
5
>>> a = 1 + 2 * 3
>>> a = 1
>>> b = 2
>>> c = a and b or c
>>> c = int('1') + 2

Operator priority

For example, the priority of and in logical operators is higher than or

>>> a = 1
>>> b = 2
>>> c = 3
>>> a + b * c
7
>>> 1 or 2
1
>>> 1 and 3
3
>>> a or b and c
1
>>> a or (b and c)
1
>>> a or 3
1
>>> (a or b) and c
3
>>> (a or b) and (c + 1)   //Two brackets of the same level, left combined
4
>>> a = 1
>>> b = 2
>>> c = a + b    //When assignment symbols appear, combine right
>>> print(c)    
3
>>> c = a or b
>>> print(c)
1
>>> a = 1
>>> b = 2
>>> c = 2
>>> not a or b + 2 == c
False
>>> ((not a) or ((b + 2) == c))    //Priority: not > and > or
False

Writing Python code in a text file

The python script is a file with the suffix. py, which is executed through the command line "python filename.py"

Recommended IDE: pycharms, vscodes, pycharms for large projects, vscodes for learning, plug-ins recommended in vscodes: python, Terminal, Vim, vsCode icons

Notes

For single line annotation#
For multiline annotation```

Process control statement

Mainly conditional control (if else), circular control (for while), branch

Conditional control (if else)

# encoding: utf-8

mood = False

if mood :
    print('go to left')
    # print('back away')
# print('back away')
else :
    print('go to right')
    
a = 1
b = 2
c = 2
# if can be followed by not only a Boolean value, but also an expression
if a or b + 1 == c :
    print('go to left')
    # print('back away')
# print('back away')
else :
    print('go to right')

# encoding: utf-8
"""
    //A small program
"""
# Constant constant recommends all uppercase
ACCOUNT = 'hughie'
PASSWORD = '123456'
# python variables are recommended to be lowercase, separated by underscores, and named without humps
print('please input account')
user_account = input()

print('please input password')
user_password = input()

if ACCOUNT == user_account and PASSWORD == user_password:
    print('success')
else:
    print('fail')

# encoding: utf-8
# snippet fragment

if condition:
    pass
else:
    pass


a = True
if a:
    # pass empty statement / occupation statement
    pass
else:
    print('')


if True:
    pass
if False:
    pass

# Nested branch
if condition:
    if condition:
        pass
    else:
        pass
else:
    if condition:
        pass
    else:
        pass

# Code block
if condition:
    code1
        code11
        code22
            code333
            code444
                code5555
                code6666
    code2
    code3
else:
    code1
    code2
    code3

Overwrite with elif

# encoding: utf-8
"""
a = x

a = 1 print('apple')
a = 2 print('orange')
a = 3 print('banana')

print('shopping')
"""

a = input()
print('a is' + a)
if a == 1:
    print('apple')
else:
    if a == 2:
        print('orange')
    else:
        if a == 3:
            print('banana')
        else:
            print('shopping')


# Overwrite with elif
a = input()
print(type(a))
print('a is ' + a)
a = int(a)
if a == 1:
    print('apple')
elif a == 2:
    print('orange')
elif a == 3:
    print('banana')
else:
    print('shopping')

Loop (while for)

# encoding: utf-8
# loop

# Loop statement

# while     for
# CONDITION = True
# while CONDITION:
#     print('I am while')

counter = 1
# Recursive common while
while counter <= 10:
    counter += 1
    print(counter)
else:
    print('EOF')

# encoding: utf-8

# It is mainly used for traversing / looping sequences or collections, dictionaries
# a = ['apple', 'orange', 'banana', 'grape']
# for x in a:
#     print(x)

# a = [['apple', 'orange', 'banana', 'grape'], (1, 2, 3)]
# for x in a:
#     for y in x:
#         # print(y, end='')
#         print(y)
# else:
#     print('fruit is gone')


# a = [1, 2, 3]

# for x in a:
#     if x == 2:
#         # break will terminate when x==2 and print out 1
#         # break
#         # continue to skip when x==2 and print out 1, 3
#         continue
#     print(x)
# else:
#     print('EOF')


a = [['apple', 'orange', 'banana', 'grape'], (1, 2, 3)]
for x in a:
    # if 'banana' in x:
    #     break
    for y in x:
        if y == 'orange':
            # After the internal loop jumps out, the external loop is still executing
            break
        print(y)
else:
    print('fruit is gone')

# encoding: utf-8
# for (i=0; i<10; i++){}

# The above for loop is implemented in python
# for x in range(0, 10):
#     # range(0,10) represents 10 numbers starting from 0, excluding 10
#     print(x)


# for x in range(0, 10, 2):
#     # range(0,10,2) 2 for step size
#     print(x, end=' | ')
#     # Print result: 0 | 2 | 4 | 6 | 8|


for x in range(10, 0, -2):
    print(x, end=' | ')
    # Print results: 10 | 8 | 6 | 4 | 2|

# encoding: utf-8

a = [1, 2, 3, 4, 5, 6, 7, 8]

# for i in range(0, len(a), 2):
#     print(a[i], end=' | ')
    # 1 | 3 | 5 | 7 | 

b = a[0:len(a):2]
print(b)
    # [1, 3, 5, 7]