Introduction to variables and data types in Python

1. Variables and data types

1.1 Variables

1． Each variable stores a value—the information associated with the variable.

2. Variables can be not only integers or floating point numbers, but also strings, and can be of any data type.

1.1.1 Naming and using variables

Variable names can only contain letters, numbers and underscores, and cannot begin with numbers. Variable names cannot contain spaces, but can be separated by underscores. Python keywords and function names cannot be used as variable names. Variable names should be short and descriptive. Be careful with the lowercase l and uppercase O, as they can be mistaken for the numbers 1 and 0.

1.1.2 Avoid naming errors when using variables

The interpreter will provide a traceback when an error occurs. A traceback is a record that points out where trouble occurred.

1.2 String str

1.String is a series of characters. It is a data type. In Python, all strings enclosed by quotes can be single quotes or double quotes.

2. The Unicode standard is also constantly evolving, but the most commonly used one is to use two bytes to represent a character (if you want to use very remote characters, you need 4 bytes). Modern operating systems and most programming languages ​​support Unicode directly. Convert Unicode encoding into "variable length encoding" UTF-8 encoding.

3.Python uses single quotes or double quotes with b prefix to represent bytes type data: x = b'ABC'. Str expressed in Unicode can be encoded into specified bytes through the encode() method.

'ABC'.encode('ascii')

b'ABC

Conversely, if we read a byte stream from the network or disk, the data read is bytes. To change bytes into str, you need to use the decode() method:

b'ABC'.decode('ascii')

'ABC

4. For the encoding of a single character, Python provides the ord() function to obtain the integer representation of the character, and the chr() function converts the encoding to the corresponding Characters:

##>>> ord('A')

65

>> > ord('中')

20013

>>> chr(66)

'B'

>> > chr(25991)

'文'

5. To calculate how many characters str contains, you can use the len() function, The len() function calculates the number of characters in str. If it is changed to bytes, the len() function calculates the number of bytes. It can be seen that a Chinese character usually occupies 3 bytes after UTF-8 encoding, while an English character only occupies 1 byte.

1.2.1 Operations on strings

1. Methods are operations that Python can perform on data.

2.title() displays each word with the first letter capitalized, that is, changes the first letter of each word to capitalized.

3.upper() changes the string to all uppercase. lower() changes the string to all lowercase.

4. If the string has at least one letter, and all letters are uppercase or lowercase, the isupper() and islower() methods will return the Boolean value True accordingly. Otherwise, the method returns False.

5.salpha() returns True if the string only contains letters and is not empty;

6.isalnum() returns True if the string only contains letters and numbers and is not Empty;

7.sdecimal() returns True if the string only contains numeric characters and is not empty;

8.sspace() returns True if the string only contains spaces, spaces table character and line break, and is not empty;

9.istitle() returns True if the string only contains words starting with an uppercase letter and followed by lowercase letters.

10. The startswith() and endswith() methods return True if the string they are called starts or ends with the string passed in by this method. Otherwise, the method returns False.

11. The join() method is called on a string, the parameter is a list of strings, and a string is returned.

##>>> ', '.join(['cats', 'rats', 'bats'])12. The split() method does exactly the opposite: it is called on a string and returns a list of strings. You can also pass a split string to the split() method and specify it to split according to different strings.

'cats, rats, bats'

>>> ' '.join(['My', 'name', 'is', 'Simon'])

'My name is Simon'

>>> 'ABC'.join(['My', 'name', 'is', 'Simon'])

'MyABCnameABCisABCSimon'

##>>> 'My name is Simon'.split()

13. The rjust() and ljust() string methods return padded versions of the strings on which they are called, with spaces inserted to align the text. The first argument to both methods is an integer length used to align the strings. The second optional argument to the rjust() and ljust() methods specifies a fill character to replace the space character.

['My', 'name', ' is', 'Simon']

##>>> 'Hello'.rjust(20, '*')

'******* ********Hello'

>>> 'Hello'.ljust(20, '-')

'Hello-------- -------'

14.center() string method is similar to ljust() and rjust(), but it centers the text, Instead of left or right alignment.

15.sort() sorts strings.

16. Please be sure to note that Python programs are

case sensitive. If the wrong case is entered, the program will report an error.

17. Merge - Python uses the plus sign + to merge strings

18. Can be before the quotation mark at the beginning of the string Add r to make it a raw string. "Raw string" completely ignores all escape characters and prints all backslashes in the string.

##first_name = "ada"
last_name = "lovelace"
full_name = first_name + " " + last_name
print( full_name)
ada Lovelace

##>>> print(r'That is Carol\'s cat.')19. Delete blanks: method rstrip() right lstrip() left strip() both sides

That is Carol\'s cat.

20.

Syntax Error:

is an error that is encountered from time to time. In a string enclosed in single quotes, if an apostrophe is included, it will cause an error. Double quotes won't.

21.Print() prints, the comma will be one space empty.

22. The pyperclip module has copy() and paste() functions that can send text to or receive text from the computer's clipboard.

23. String has a replace() method

##>>> a = 'abc'>>> a.replace('a', 'A')The null value is a special value in Python, represented by None. None cannot be understood as 0, because 0 is meaningful, and None is a special null value.

'Abc'

##1.2.2 empty Value

1.2.3 Constants

Constants are variables that cannot be changed. For example, the commonly used mathematical constant π is a constant. In Python, variable names in all uppercase letters are usually used to represent constants: PI = 3.14159265359

##1.2.4 Assignment

In Python, the equal sign = is an assignment statement, and any data type can be assigned a value To a variable, the same variable can be assigned repeatedly, and it can be a variable of different types:

a = 123 # a is an integer

print (a)a = 'ABC' # a becomes a string

print(a)

Assignment statement : a, b = b, a + b

##t = (b, a + b) # t is a tuple

a = t [0]b = t[1]

1.2.5 Formatting

There are two types of formatting in Python The first one is implemented with %, and the second one is { } format.

'Hello, %s' % 'world'

%?, the brackets can be omitted.

% operator is Used to format strings. Within the string, %s means replacing with a string, %d means replacing with an integer, there are several %? placeholders, followed by several variables or values, and the order must be corresponding.

If there is only one

Commonly used placeholders:

##%d Integer%f Floating point number

%s String%x Hexadecimal integer

Among them, formatted integers and floating point numbers can also specify whether to add 0 And the number of digits in integers and decimals:

#>>> '%2d-%02d' % (3, 1)

' 3-01'

>>> '%.2f' % 3.1415926'3.14'1. Normal use

If you are not sure what to use, %s will always work, it will convert any data type to a string.

Sometimes, the % in the string is an ordinary character and needs to be escaped. Use %% to represent a %.

The second formatting method, format, replaces % with {}.

##>>> print("My name is {} and I am {} years old this year".format ("Xiao Li", 20))

My name is Xiao Li, I am 20 years old this year

2. You can also modify the formatting order by filling in numbers in brackets

##>>> print("My name is {1 }, this year is {0} years old".format("Xiao Li", 20))

My name is 20, this year Xiao Li is years old

3. Get variables through key

##>>> print("My name is {name} and I am {age} years old this year". format(name="Xiao Li", age=20))1.2.6 Escape Characters

My name is Xiao Li, I am 20 years old this year

Whitespace - refers to any non-printing characters such as spaces, tabs, and newlines.

Escape character\ can escape many characters\t tab character \n newline

character\ itself also needs to be escaped, so the character represented by \\ is \

If there are many characters in the string that need to be escaped, you can use r'' in Python to represent ''. The internal string is not escaped by default:

>>> print('\\\t\\')1.3 Number

\ \

>>> print(r'\\\t\\')

\\\t\\

1.3.1 Integer int

can be performed Arithmetic.

Since computers use binary, it is sometimes more convenient to use hexadecimal to represent integers. Hexadecimal is represented by the 0x prefix and 0-9, a-f, for example: 0xff00, 0xa5b4c3d2, etc.

Division of integers

is exact. In Python, there are two kinds of division. One division is /, /The result of division is a floating point number. Even if two integers are exactly divisible, the result is a floating point number. Another kind of division is //, called floor division. The division of two integers is still an integer. % Take the remainder.

1.3.2 Floating point number float

Python calls numbers with decimals floating point numbers. The reason why they are called floating point numbers is because when expressed in scientific notation, a floating point number The decimal point position is variable. For example, 1.23x10

9

and 12.3x10⁸ are completely equal. For very large or small floating point numbers, they must be expressed in scientific notation. Replace 10 with e. 1.23x109 is 1.23e9, or 12.3e8, 0.000012 can be written as 1.2e-5, etc.

1.3.3 Use the function str() to avoid errors

Data type checking can be implemented using the built-in function isinstance():

def my_abs(x):

if not isinstance(x, (int, float)):

raise TypeError('bad operand type')

if x >= 0:

                                                                        ’ s ’ ’ through through through through through through through through out out off off out off out through out through out out through out right Through out through over over over ‐ After ‐ ‐ n w w w w‐ ‐ to to to to to to to to to come a 1.4 Notes

#1. Statements starting with # are comments. Comments are for human viewing and can be any content. The interpreter will ignore the comments. Each other line is a statement, and when the statement ends with a colon:, the indented statement is considered a code block.

#. . . . . .

2. Since the Python source code is also a text file, when your source code contains Chinese, you need to Be sure to specify saving as UTF-8 encoding. When the Python interpreter reads the source code, in order for it to be read in UTF-8 encoding, we usually write these two lines at the beginning of the file:

#!/usr/bin/env python3

# -*- coding: utf-8 -*-

3.Document string Comments ””” ”””1.5Zen of Python

The Zen of Python, by Tim Peters

Beautiful is better than ugly.

Explicit is better than implicit.

Simple is better than complex.

1. Complex is better than complicated.

2. Flat is better than nested.

3. Sparse is better than dense.

4. Readability counts.

5. Special cases aren't special enough to break the rules.

6. Although practicality beats purity.

7. Errors should never pass silently.

8. Unless explicitly silenced.

9. In the face of ambiguity, refuse the temptation to guess.

10. There should be one-- and preferably only one --obvious way to do it.

##Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

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