Simply understand , An identifier is a name , It's like each of us has his own name , Its main function is as a variable 、 function 、 class 、 Names of modules and other objects .
Order rules
Name of identifier , In addition to following these rules , Identifiers in different scenarios , Its name also has certain norms to follow
Reserved word is Python Some words in a language that have been given a specific meaning , This requires developers to develop programs , You can't use these reserved words as identifiers for variables 、 function 、 class 、 Templates and other object naming .
>>> import keyword
>>> keyword.kwlist
['False', 'None', 'True', 'and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield']
Python The functions that come with the interpreter are called built-in functions , These functions can be used directly , There is no need to import a module .
>>> dir(__builtins__)
a,b=1,2
There are two characteristics of weakly typed languages ：
Python There are four types of values for median numbers ： Integers 、 long integer 、 Floating point and complex number .
integer , Such as 1, -1, 0
Long integer , It's a big integer , Such as 234234243324
floating-point , Such as 1.23、3E-2
The plural , Such as 1 + 2j、 1.1 + 2.2j
stay Python View types in , Use type() Built in functions .
>>> type(2)
int
>>> type("2")
str
Hexadecimal : stay Pthon Use in 0x At the beginning “ Numbers ” Represents a number in hexadecimal . about 9 Later numbers , There is no corresponding Arabic numeral . Then use a Express 10 , b Express 11 , By analogy , Last f Express 15. Do not distinguish in hexadecimal expression “ Letter ” The case of
octal : stay Python Use in 0o The first number represents octal , Be careful 0 Followed by English letters o
Binary system :0b The switch expression represents binary . In binary expressions , Only numbers... Can be used 0 And 1
hex1 = 0x45
hex2 = 0x4Af
print("hex1Value: ", hex1)
print("hex2Value: ", hex2)
hex1Value: 69
hex2Value: 1199
int(x) take x Convert to an integer .
float(x) take x Converts to a floating point number .
complex(x) take x To a complex number , The real part is x , The imaginary part is 0 .
>>> a = 3.1415926
>>> int(a)
3
## Convert to a complex expression .
>>> complex(a)
(3.1415926+0j)
‘/’ The result of division is a decimal , Floating point numbers
# Whether the operand is an integer or a floating point number , The result is always floating point numbers
>>> 4.0 / 2
2.0
>>> 1 / 1
1.0
If you want to discard the decimal part , That is, perform an integer division operation , Double slashes can be used // .
# Whether the operand is an integer or a floating point number , The quotient obtained does not retain the mantissa of the floating-point number
>>>4//2
2
>>>5.0//2
2.0
Seeking remainder （ modulus ） Operator ,x % y As the result of the x Divide y The remainder of
>>> 10 % 3
1
>>> 2.75 % 0.5
0.25
>>> 10 % -3
-2
>>> 10 // -3
-4
chengfang （ Exponentiation ） Operator . Please note that , The priority of the power operator is negative （ Monocular subtraction ） high , therefore -32 Equivalent to -(32) . If you want to calculate (-3)**2 , Must be specified in parentheses .
>>> -3 ** 2
-9
>>> (-3) ** 2
9
# In operation two Integer or floating-point values , + Is the addition operator . however , When used with two strings , It will string Connect , Become “ String connection ” The operator .
>>> 'Alice' + 'Bob'
'AliceBob'
# If you use the plus operator on a string and an integer value ,Python I don't know how to deal with
>>> 'Alice' + 42
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-5-4e9a9e71481a> in <module>
----> 1 'Alice' + 42
TypeError: can only concatenate str (not "int") to str
## When used with two integer or floating-point values ,* The operator represents multiplication . but * The operator is used for a string Value and an integer value , It became “ String copy ” The operator .
>>> 'Alice' * 5
'AliceAliceAliceAliceAlice'
>>> var2 = "Runoob"
>>> print ("var2[1:5]: ", var2[1:5])
var2[1:5]: unoo
#in The usage of the keyword is similar to that of the operating system , Used to determine whether an element is in a list or string .
>>> if( "H" in a) :
>>> print("H In variables a in ")
>>> else :
>>> print("H Not in variable a in ")
H In variables a in
#not Keyword is used to negate
>>> if( "M" not in a) :
>>> print("M Not in variable a in ")
>>> else :
>>> print("M In variables a in ")
M Not in variable a in
The usage is to insert a value into a string format character %s In the string of .
>>> " My name is %s, This year, %d year !" % (' Xiao Ming ', 10)
' My name is Xiao Ming , This year, 10 year !'
Python String formatting symbols :
%s ： Formatted string ;
%d ： Formatted integer ;
%f ： Formatted floating point number , Precision after the decimal point can be specified ;
Specify decimal precision
%m.nf
%.nf
m Represents the minimum width ,n Indicates output accuracy ,. Must exist .
f = 3.141592653
# Minimum width is 8, After the decimal point 3 position
print("%8.3f" % f)
# Minimum width is 8, After the decimal point 3 position , Left complement 0
print("%08.3f" % f)
# Minimum width is 8, After the decimal point 3 position , Left complement 0, Signed
print("%+08.3f" % f)
Single and double quotes , Most of the cases are exactly the same
'Let's go!'
Report errors
# The backslash （ \ ） Escape Quotes
'Let\'s go!'
Escaping characters can sometimes cause problems , For example, I would like to express an inclusion Windows route D:\Program Files\Python 3.8\python.exe Such a string , stay Python It must not be possible to write this directly in the program , Whether it's a normal string or a long string . because \ The particularity of , We need to look at each of the strings \ All of them are escaped , It's written as D:\Program Files\Python 3.8\python.exe This is the form .
Add... At the beginning of a normal or long string r Prefix , It becomes the original string , The specific format is ：
str1 = r' Original string content '
str2 = r""" Original string content ""
rstr = r'D:\Program Files\Python 3.8\python.exe'
print(rstr)
https://www.liaoxuefeng.com/wiki/1016959663602400/1017075323632896
Python There may be identifiers and strings in several languages in the program code to display 、 Output or comment . To store and display these different languages , Different countries and organizations have developed several character set standards . Common character sets are ASCII Character set 、 Simplified Chinese GB2312 Character set 、 Traditional Chinese Big5 Character set 、 Simplified Chinese GB18030 Character set 、Unicode Character set, etc. .
stay Python3 in , All strings are Unicode character string , stay Unicode There are many ways to put numbers 23383 Expressed as data in the program , Include UTF-8、UTF-16、UTF-32
In computer memory , Unified use Unicode code , When you need to save to a hard disk or need to transfer , Just switch to UTF-8 code .
[ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-D05wg0Cx-1656062146825)(https://boostnote.io/api/teams/dlm1ZMj21/files/0971c779c2cf97bc40e136e29a66b04b029eb5ef84f40cae80904530464c4d1c-image.png)]
Python Three quotes allow a string to span multiple lines , String can contain line breaks 、 Tabs and other special characters
>>> para_str = """ This is an example of a multiline string >>> Multi line strings can use tabs >>> TAB ( \t ). >>> You can also use line breaks [ \n ]. >>> """
>>> print (para_str)
bytes yes Python 3.x New type , stay Python 2.x There is no such thing as .
If you use the appropriate character set , A string can be converted to a byte string ; In turn, , The byte string can also be restored to the corresponding string .
# Using the string of encode() Method code into bytes, By default utf-8 Character set
a= “ Study ”.encode(‘utf-8’)
# call bytes Object's decode() Method to decode it into a string
st= a.decode(‘utf-8’)
# Continuous assignment
A=B=C=100
# The extended assignment operator
x += y # Be similar to x = x + y
x -= y # Be similar to x = x - y
= and == Are two different operators ,= Used to assign , and == Used to judge whether the values on both sides are equal
exp1 if contion else exp2
condition It's the judgment condition ,exp1 and exp2 It's two expressions . If condition establish （ The result is true ）, Is executed exp1, And put exp1 As the result of the entire expression ; If condition Don't set up （ The result is false ）, Is executed exp2, And put exp2 As the result of the entire expression .
4+4<<2
# Be similar to (4+4) << 2
100 / 25 * 16,/ and * Same priority for , Which one should be executed first ？
The so-called Associativity , That is, when multiple operators with the same priority appear in an expression , Which operator to execute first ： First execute the left one, which is called left associativity , First execute the on the right, which is called right associativity .
