Python Learning---------Day2

The fourth chapter introduces Python object types
Why use built-in types
Built-in objects make programs easier to write
Built-in objects are extended components
Built-in objects tend to be more efficient than custom data structures
Built-in objects are part of the language standard
Python's core data types
Digital
String
List
Dictionary
Meta-group
File
Other types (set, type, None, Boolean)
Digital
Import Math
Math.PI
MATH.SQRT (54)
Import Random
Random.random ()
Random.choice ([1,2,3,4])
String
Sequence operations
s= ' spam '
Len (s)
S[0]
S[1]
S[len (s) -1]== s[-1]
S[-2]
S[1:3] # shard operation
S[:-1]
s[:] #就是ｓ
S+ ' xyz ' #字符串支持加号合并, a new string is synthesized from a string
S*8
The string is immutable--the value cannot be changed after its creation, for example, you cannot pass a
Changes the string, but you can always create a new string to assign the same variable name to it.
Value
Type-specific methods
S.find (' pa ') #函数返回传入子字符串的偏移量, not found in case of return-1
S.replace (' pa ', ' xyz ') #搜索并进行替换
Line= ' Aaa,bbb,ccccc,dd '
Line.split (', ')
s= ' spam '
S.upper () #大写
S.isalpha () # Judging is not the letter
Line=line.restrip () #去除换行符
Dir (S) can find all properties of an object
Ord (' A ') returns the ASCII code of a
Python allows strings to be included in single or double quotes, and it can also represent multiple lines of string in three quotation marks.
Form
Pattern matching
Import re
Match=re.match (' hellp[\t]*. *) World ", ' Hello Python World ')
Match.group (1)

Match=re.match ('/(. *)/(. *)/(. *) ', '/usr/home/lqs ')
Match.Groups ()
A list is an ordered collection of positions related to an arbitrary type of object, it does not have a fixed size, does not want a string, its
Size is Variable
l=[123, ' spam ', 1.23]
Len (L)
L[0]
L[:-1]
l+[4,5,6] #创建了一个新的列表
L # no change
L.append (' NI ')
L.pop (2)
m=[' BB ', ' AA ', ' CC '
M.sort ()
M.reverse ()
Bounds checking Python does not allow referencing elements that do not exist
Nesting
m=[[1,2,3],
[4,5,6],
[7,8,9]]
List parsing
COL2=[ROW[1] for fow in M]
[row[1] for row im M if row[1]%2==0]
Diag=[m[i][i] for i in [0,1,2]]
Doubles=[c*2 for C in ' spam ']
Dictionary
d={' food ': ' spam ', ' Qian ': 4, ' Color ': ' Pink '}
d[' food ']
d[' Qian ']+=1 dictionary with variability
a={}
a[' name ']= ' Bob '
a[' age ']=23
Re-visit nested
rec={' name ': {' first ': ' Bob ', ' last ': ' Snuth '},
' Job ': [' dev ', ' Mgr '],
' Age ': 40.5}
rec[' name ']
rec[' name ' [' Last ']
rec[' job '].append (' janitor ')
D={' A ': 1, ' B ': 2, ' C ': 3}
Ks=d>keys ()
Ks.sort ()
For key in KS:
Print key, ' = = ', D[key]

For key in Sorted (D):
Print key, ' = = ', D[key]
Iteration and optimization
Square=[x**2 for x in [1,2,3,4]]
Square=[]
For x in [1,2,3,4]:
Square.append (x**2)
Non-existent key if test
D.has_key (' F ')
If not D.has_key (' F '):
print ' missing '
Tuples are immutable sequences
T= (a)
File
F=open (' Data.txt ', ' W ')
F.write (' hello\n ')
F.write (' world\n ')
F.close ()
F=open (' Data.txt ')
Bytes=f.read ()
Print bytes
Bytes.split ()

Fifth chapter number
Numeric constants
Hybrid Type automatic upgrade
Traditional division, Floor division, and true Division
The built-in int function transforms a string of numbers into an integer, and can be defined by a second parameter to
The conversion of the number into the system
Int (' 0100 ', 8) int (' 0x40 ', 16)
Eval (s) converts an octal or hexadecimal string into a normal integer, but more overhead can cause
Security issues
Int (2.56) round (2.567) round (2.567,2)
Decimal number
From decimal Import Decimal
Decimal (' 0.1 ') +decimal (' 0.1 ') +decimal (' 0.1 ')-decimal (' 0.3 ')

The sixth chapter dynamic type Introduction
A=3
Create an object to represent the value 3
Create a variable A, if it has not yet been created
Connect the variable with the new object 3

A variable is an element of a system table that has a connection to an object
The object is a piece of memory allocated, and there is enough space to show the value that they represent
A reference is an automatically formed pointer from a variable to an object
Note issues with shared references
a=[1,2,3]
B=a
a=2

a=[1,2,3]
B=a
a[0]=2

a=[1,2,3]
b=a[:]
a[0]=2
Problem solving for dictionary shared references using the copy Deepcopy method

l=[1,2,3]
M=l
L==m TRUE
L is M TRUE
= = Tests that two referenced objects are sufficient to have the same value
The IS operator is the identity of the check object

Python Learning---------Day2