Day3 & Day4 Python Study Notes & Key Points of Attention

Part 1: Python study notes

===================

4.4. Dict dictionary table

4.4.1 Example 1: {statement}

• Is defined: d = { 'ISBN': '234234', 'Title': 'Python Started', 'Price': 39.00}
• Display value: d [ 'Title'] -> 'Python entry'
• d['Price'] --> 39.00
• Append: d [ 'Author'] = 'Jerry' Note: for the dictionary table does not exist in the key will be automatically added, but not such a list operation
• Key obtained: d.get ( 'Price') -> 39.00
• Return did not get the key: d.get ( 'price', 0.0) -> 0.0
• Support situ changes: d [ 'Price'] = 99.00

4.4.2 Example 2: function declaration

• 定义：emp = dict(name='Mike',age=20,job='Dev')
• len(emp) --> 3
• . Merge operation: 1 dep = { 'department': 'Technology'}; 2. emp.update (dep)
• Eject key: emp.pop ( 'age') -> 20

4.4.3. Statement

• {Key: value, ...}
• dict (key = value)

4.4.4 Operation

• Obtain
  • D [ 'key']
  • d.get ( 'key', the default)
• merge
  • d.update(d2)

4.4.5. Properties

• keys()
• values()
• items() : Key+value
• Supports nested: emp = { 'age': 20, 'name': { 'firstname': 'Jerry', 'lastname': 'lee'}, 'dept': 'Administration'}

4.4.6. Sort key

• The keys () into the list
  • ks = list(d.keys())
  • ks.sort()
• Use global function sorted ()
  • ks = d.keys()
  • for k in sorted(ks):
  • Print(k,d.get(k))

4.5. Tuple tuple

4.5.1. Characteristics

• Ordered set of arbitrary objects
• Access by the subscript
• It is "immutable" type
  • t = (1,2,3,4,5)
  • t [0] = 99 -> given
• Length is fixed, of any type, any nested

4.5.2. Statement

• (Element) i.e., (1,2)
  • 1,2 omit statement () of
• index (val): Find index
• (Val) count: the number of statistics

4.5.3 Calculation

• (1,2)+(3,4) --> (1,2,3,4)

4.5.4. Assignment

• x = (40,) --> (40,)
• x = 40, --> (40,)
• len (x) -> 1

4.5.5.Named tuple

• from collections import namedtuple
• Employee = namedtuple('Employee',['name','age','department','salary'])
• jerry = Employee ( 'Jerry', age = 30, department = 'Finance Department', salary = 9000.00)
• jerry.name --> 'Jerry'
• jerry.salary --> 9000.0

4.6 File

4.6.1. The basic syntax

• file = open ( 'filename ", mode)
• mode
  • r Reading
  • w write
  • a Append
  • b binary
  • 示例：f = open('data.bin','rb').read()
  • + Can read, but also write
• Examples
  • Hello.txt a newly created file: myfile = open ( 'hello.txt', 'w')
    • myfile.write ( 'Youpin class \ n') -> 5
    • myfile.write('Hello world!\n') --> 13
  • A reading of file: f = open ( 'hello.txt')
    
    • reached, f.read () reads the entire contents of which, corresponding to the pointer
    • f.readline () to read each row of information
  • l = open ( 'hello.txt') readlines () -.> [ 'excellent product class \ n', '! Hello world \ n']
    
    • for line in l:
    • print(line)
  • Chinese showed normal: f = open ( 'course.txt', 'w', encoding = 'utf8')
    
    • f.write ( 'class Youpin Python Tutorial \ n') -> 14
    • f.write('www.codeclassroom.com') --> 21
    • f.close()

4.6.2 Operation

• read()
• readline()
• readlines()
• close()
• Examples
  • x,y,z = 1,2,3
  • l = [1,2,3]
  • f = open('datafile.txt','w')
  • f.write('{},{},{}'.format(x,y,z)) -->5
  • f.write(str(l)) -->9
  • f.close()

4.6.3.pickle access Python objects

• dump (objects, object files)
• load (file)
• Example 1: d = { 'a': 1, 'b': 2}
  • f = open('datafile.pkl','wb')
  • import pickle
  • pickle.dump(d,f)
  • f.close()
  • open('datafile.pkl','rb').read()
• Example Two: f = open ( 'datafile.pkl', 'rb')
  • data = pickle.load(f)

4.7. Summary

4.7.1. Collection

• sequence
  • variable
    • List list
  • Immutable
    • String str
    • Tuple tuple
    • Byte array
• Mapping
  • Dict dictionary table
• set
  • Set

4.7.2. Digital

• Integer
  • int
  • boolean
• Float
  • float
  • Decimal
  • ... scores

4.7.3. Callable

• Function function
• Generator Generator
• Class Class
• Method Method

4.7.4. Other

• file
• None
• view
• ...

4.7.5. Internal

• Type
• ...

5. statements, expressions and flow control

5.1. Code style

5.1.1. Code Format Guide

• PEP8
• Indent 4 spaces
• No more than 79 characters in a row
• Blank line

5.2. Assignment statements

5.2.1 Example 1: x = 5

5.2.2 Example 2:. (X, y) = (5,10)

• or x,y = 5,10

5.2.3 Example 3:. [X, y, z] = [10,20,30]

• Exchange: x, y = y, x
• or [a,b,c]=(1,2,3)

5.2.4 Example 4:. A, b, c = 'uke'

• a --> 'u'
• b --> 'k'
• c --> 'e'

5.2.5. Basic

5.2.6. Sequence assignment

• Example 1: a, b, c = 'you' no problem
  • a, b, c = 'youpin' error
  • 1 modified to
    
    • a,b,c = s[0],s[1],s[2:]
  • 2 or modify a, b, * c = s

5.2.7 The extended sequence unpacking assignment

• * Variable, get the remaining elements to list
  • Example: s = 'youpin'; a, b, * c = s

5.2.8. Multi-target assignment

• a = b = c = 'week'

5.2.9. Assignment parameterization

• x = 5; y = 6; x = x + y; x --> 11
• a,b = 1,2; a += b; a --> 3
• + = Or extend () to expand the list of values

5.3. Expression

5.3.1. Function Call

5.3.2. Method Invocation

5.3.3. Literals

5.3.4. Print Operations

• print()
  • sep = 'separator'
  • end = 'terminator'
  • file = specified file
    
    • 示例：print(s,url,url2,end='......\n',file=open('result.txt','w',encoding='utf8'))

5.4. Process control

5.4.1.if .. statement

• The general format
  • score = 75
  • if score >= 60:
  • print ( 'pass')
  • else:
  • print ( 'fail')
• Achieve multiple branch
• Ternary operator
  • a = Y if X else Z
    • Example: result = 'pass' if score> = 60 else 'fail'

5.4.2.while cycle

• The general format
  • x = 'youpinketang'
  • while x:
  • print(x,end=' ')
  • x = x[1:]
• break out of
• continue jump head cycle, cycle performed physiognomy
• pass placeholder
• else

5.4.3.for cycle

• The general format
  • for x in target sequence:
    • Example: for x in [1,2,3,4]
  • Subsequent statements pass
    
    • Example: print (x, end = '')
• range () Returns the generated object, not a list
  • for x in range(1,100):
• enumerate()
  • s = 'youpinketang'
  • for (idx, item) in enumerate(s):
  • print('{},{}'.format(idx + 1,item))