What is a variable? Before learning variables, let's look at some of the things you need to know:
a = 1 b = 2 c = a + b print(c) print(1+2) Copy Code
Here we implement an addition operation. It is also possible to output 1+2 directly with the output statement print, but if we use variables, then one variable equals 1 and the other variable equals 2, which you can call assignment. This means giving a value to a variable.
So a, B and c are variables here, and 1 and 2 are the values we give to the variables, giving them a value. c is the medium we use to implement a+b, and of course you can print directly (a+b). They are written differently and get the same result.
Variables also have their own temper to see what they don't allow: valid naming invalid naming
china china-t(No dashes allowed) chinaHu china hu(No spaces allowed) china_hu 1china(Number start is not allowed) _china china$(Special characters are not allowed) CHINA 'china'(String form is not allowed) china9 Copy Code
If this form doesn't make sense, what about this?
abc_xyz = 'Hello Python' #Legal. HelloWorld = 'Hello Python' #Legal. abc = 'Hello Python' #Legal. xyz#abc = 'Hello Python' #"Not legal, not allowed in identifier"#"Number. abc1 = 'Hello Python' #Legal. 1abc = 'Hello Python' #Illegal, identifier does not allow number start. w b = 'Hello Python' #Illegal, with spaces in between The quotation marks above are strings, which will be explained in the next chapter, so you can understand them a little. Copy Code
Be careful:
1,Whether it's a variable name or a file name, try to keep it short or descriptive for yourself or others to see. For example: name than n Good number, first_name than f_n good 2,And don't put Python Use the keyword or function name as the variable name For example, the most obvious is print,class etc. 3,Try not to use lowercase letters l With capital letters O,I might get confused one day and think of it as a number 1 or 0 4,Note the Chinese and English characters, especially " ''These two strings
Character string
What is a string? Important? The answer is yes, it's so important that you need it later, so go ahead and learn.
``` name = 'Hello World' print(name) ``` Copy Code
Here'Hello World'is what we call a string. Name is its variable. A variable name equals a string followed by a single quotation mark and double quotation marks. Whether it's a Chinese character, a letter, or a number, it's a string.
Notes for single and double quotation marks:
``` # Correct Writing name = '"There is nothing else to do in Qing An"' # Wrong Writing name1 = ''There is nothing else to do in Qing An'' ``` Copy Code
Let's make some changes to the string:
.title()
``` name = 'hello python' print(name.title()) ``` Copy Code
Here we get a string with the first letter capitalized:
Hello Python Copy Code
.upper()
``` name = 'hello python' print(name.upper()) ``` Copy Code
Here we get a string with all uppercase letters:
``` HELLO PYTHON ``` Copy Code
.lower() method
Here our strings are guaranteed to be all or part uppercase first, otherwise they will print out exactly the same as the strings we write in!
``` name = 'HELLO PYTHON' print(name.lower()) ``` Copy Code
Here we get a string with all lowercase letters:
hello python Copy Code
strip() method
Remove spaces: Note: Tab \t can also be spaces: print('\tpython')
``` name = 'python ' name = ' python' name = ' python ' # Remove trailing spaces print(name.rstrip()) # Remove leading spaces print(name.lstrip()) # Delete spaces on both sides print(name.strip()) ``` Copy Code
The result is:
``` python python python ``` Copy Code
Next, let's use some variables in the string to see an example:
first_name = 'Hello' last_name = 'World' # f is the meaning of a string, short for format, is a format #Many errors can be avoided print(f"{first_name}{last_name}") # You can also write that first_name = 'Hello' last_name = 'World' # Given a variable, full_name = f"{first_name}{last_name}" # Here the first two defined variables are assigned to the new variable # Print by a new variable, and print as described above print(full_name) ``` Strings are also useful for many purposes: can you avoid a lot of errors!!! Copy Code
first_name = 'Qing An' last_name = 'Nothing else' # You can output some Chinese characters or English numbers directly in it. print(f"1,My name is:{first_name}{last_name}!") ``` Copy Code
The result of these three examples is:
``` Hello World Hello World There is nothing else to do in Qing An ``` Copy Code
It's still interesting to look closely, so let's take a look at the value.
clear | security | nothing | other | matter | |
positive sequence | 0 | 1 | 2 | 3 | 4 |
Reverse order | -5 | -4 | -3 | -2 | -1 |
Name ='nothing else in peace'--> print (name[:3])
Print (name [2:4]) --> take value: no difference # positive sequence
Print (name[-3:-1]) --> take value: no difference # Reverse Order
print(name[-2:]) -->Value: nothing else # Reverse Order
Print (name [0:5:2]) -->Value: Nothing to do # Interval Value
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