II Notes and indents

1. Notes

• Single-Line Comments

  In Python, a single line comment starts with # and the code is as follows:

print("Hello, Python!")
#print("Hello, Python!")

The operation results are as follows:

print("Hello, Python!")

• multiline comment

Multi line comments can use multiple # numbers, as well as' 'and' ', and the code is as follows:

# First comment
# Second comment
 
'''
Third note
 Fourth note
'''
 
"""
Fifth note
 Sixth note
"""
print ("Hello, Python!")

Execute the above code and the output result is:

print("Hello, Python!")

2. Indent

The most distinctive feature of python is that it uses indentation to represent code blocks without curly braces {}.

The number of indented spaces is variable, but statements in the same code block must contain the same number of indented spaces. The code is as follows:

if True:
    print ("True")
else:
    print ("False")

Inconsistent number of spaces in the indentation number of the last line of the following code will cause a running error:

if True:
    print ("Answer")
    print ("True")
else:
    print ("Answer")
  print ("False")    # Inconsistent indentation will lead to running errors

Due to the inconsistent indentation of the above procedures, the following errors will appear after execution:

 File "test.py", line 6
    print ("False")    # Inconsistent indentation will lead to running errors
                                      ^
IndentationError: unindent does not match any outer indentation level

 

III Multiline statement writing

Python usually writes a statement on one line, but if the statement is very long, we can use backslash \ to implement multi line statements, for example:

total = item_one + \
        item_two + \
        item_three

For multiline statements in [], {}, or (), you do not need to use backslash \, for example:

total = ['item_one', 'item_two', 'item_three',
        'item_four', 'item_five']

 

IV Number type

There are four types of numbers in python: integer, Boolean, floating-point number and complex number.

• Int (integer), such as 1. There is only one integer type int, expressed as a Long integer, without Long in Python 2
• bool (Boolean), such as True, False
• float (floating point number), such as 1.23, 3E-2
• Complex (complex), such as 1 + 2j, 1.1 + 2.2j

 

V String and blank line

1. String

• Single quotation marks and double quotation marks are used exactly the same in python.
• Use three quotation marks ('' 'or' ') to specify a multiline string.
• Escape character\
• Backslashes can be used to escape, and r can prevent backslashes from escaping.. If r"this is a line with \n", it will be displayed instead of a line break.
• Concatenate strings literally, such as "this" "is" "string" will be automatically converted to this is string.
• Strings can be concatenated with the + operator and repeated with the * operator.
• Strings in Python can be indexed in two ways, starting with 0 from left to right and - 1 from right to left.
• Strings in Python cannot be changed.
• Python does not have a separate character type. A character is a string with a length of 1.
• The syntax format of string interception is as follows: variable [header subscript: tail subscript: step size]

  word = 'character string'
  sentence = "This is a sentence."
  paragraph = """This is a paragraph,
  Can consist of multiple lines"""

Example:

str='123456789'
 
print(str)                 # Output string
print(str[0:-1])           # Output all characters from the first to the penultimate
print(str[0])              # First character of output string
print(str[2:5])            # Output characters from the third to the fifth
print(str[2:])             # Output all characters starting from the third
print(str[1:5:2])          # Output every other character from the second to the fifth (in steps of 2)
print(str * 2)             # Output string twice
print(str + 'Hello')         # Connection string
 
print('------------------------------')
 
print('hello\nrunoob')      # Escape special characters with backslash (\) + n
print(r'hello\nrunoob')     # Add an r before the string to represent the original string without escape

result:

123456789
12345678
1
345
3456789
24
123456789123456789
123456789 Hello
------------------------------
hello
runoob
hello\nrunoob

2. Blank line

Empty lines are used to separate functions or class methods, indicating the beginning of a new piece of code. The class and function entry are also separated by an empty line to highlight the beginning of the function entry.

Blank lines, unlike code indentation, are not part of Python syntax. When writing, do not insert blank lines, and the Python interpreter will run without error. However, the function of blank lines is to separate two sections of code with different functions or meanings, so as to facilitate the maintenance or reconstruction of the code in the future.

Remember: blank lines are also part of the program code.

 

 

Vi User input and output

1. Input

Input function input() uses format: variable = input (Prompt string)

Example:

nunber=input("please enter a number")

2. Output

The output function print() uses the format: Print (string or string variable to be output)

print. The default output is line feed. If you want to realize no line feed, you need to add "end =" "at the end of the variable

Example:

print("what do you mean?",end="")
print("hello world")

result:

what do you mean?hello world

 

VII Use of import

In python, use ， import ， or ， from Import ， to import the corresponding module.

Import the whole module in the format of import somemodule

Import a function from a module in the format from some module import somefunction

Import multiple functions from a module. The format is: from some module import firstfunc, secondfunc, thirdffunc

Import all functions in a module in the format from some module import*