python-02 (Introductory Basics 2 - Basic Common Grammar)

1. Logical judgment words

1.1 Boolean type

1.1.1 When python is False

• Cases where the object is None and False，
  • Examples are as follows:
    
  • It should be noted that in Python, there is a constant "None", which represents a null value, not 0, nor an empty string.
    
• Empty strings and empty sequences or sets (ie: empty list[], empty dictionary{}, empty tuple()),as follows:
  
• Numeric type with value 0,as follows:
  

1.2 Logical judgment word not

• as follows:
  

2. for statement

2.1 Grammatical structure

• as follows:

  for 变量 in 序列:
  	循环要执行的动作
  

2.2 Example

2.2.1 Example 1 - Loop iterating over strings

• as follows:

  for item in "love":
      print(item)
  
      
  l
  o
  v
  e
  

  	strs = "love"
  for i in strs:
      print(i,end=' ')
  
      
  l o v e 
  

  

2.2.2 Example 2 - Performing numerical loops

2.2.2.1 Simple loop (combined with range function)

• Note that the for loop cannot iterate numeric types, int types, for example: 12345 belongs to a number, a whole, and counts as an element, as follows:
  

• If you want to print numbers in a for loop, you need to use the range function, as follows:

  for i in range(5):
      print (i)
  

  for i in range(5):
      print (i,end = ' ')
  

  sum = 0
        
  for i in range(5):
        sum = sum +i
  
        
  print (sum)
  

  
  

2.2.2.2 Other uses of range

• range(m,n)In the form, as follows:

  for i in range(1,5):
        print(i,end = ' ')
  

• range(m,n,i)In the form, as follows:
  

2.2.2.3 range summary

• The range function can get an integer within a range
  • range(n): The default starts from 0, and the range of numbers is [0,n-1];
  • range(m,n) ：Close left and open right(Including the left side but not the right side), that is, the range of numbers is[m,n-1]
  • range(m,n,i) Integer from m to n, each i is accessed once, the range is [m,n-1], where i 为步长.

2.2.3 Example 3 - Circular List

• as follows:

  dogs = ['麦兜','泡泡','大牙','贝塔']
  for dog in dogs:
      print(dog)
  

  

  	nums = [1,2,3,4,5]
        
  for num in nums:
        print(num)
  

  

2.2.4 Example 4 - Looping Tuples

• as follows:

  d = ("I","love","you")
  for i in d:
      print(i,end = ' ')
  

  

2.2.5 Example 5 - Looping through the dictionary

• Writing method 1:

  e = {
        
        '麦兜': 3,'泡泡': 5}
  for key in e:
      print(key,'===>',e[key])
  
      
  麦兜 ===> 3
  泡泡 ===> 5
  

  
• Writing method 2:

  e = {
        
        '麦兜': 3,'泡泡': 5}
  for k,v in e.items():
      print("{0}===>{1}".format(k,v))
  
      
  麦兜===>3
  泡泡===>5
  

  
• Writing method 3:

  e = {
        
        '麦兜': 3,'泡泡': 5}
  for k,v in e.items():
      print(k,':::::',v)
  
      
  麦兜 ::::: 3
  泡泡 ::::: 5
  

  

2.2.6 Example 6 - Looping through set

• as follows:

  dogs = {
        
        '麦兜','泡泡','大牙'}
          
  print(type(dogs))
          
  <class 'set'>
  
  for i in dogs:
          print(i,end = ',')
  
          
  大牙,泡泡,麦兜,
  

  

3. if statement

3.1 Several common usages

• as follows:

  if … else…
  
  if …elif…else…
  
  if not …
  
  if … not …
  

3.2 Example

3.2.1 if … else …

• Writing method 1 is as follows:

  n = 5
        
  if n > 3:
        print('n>3')
  else:
      print('n<=3')
  

  
• Writing method 2 is as follows:

  print('n>3') if n >3 else print('n<3')
  

  

3.2.2 if … elif … else…

• as follows:

  score = 86
  if score >= 95:
      print('A+')
  elif score >= 90:
      print('A')
  elif score >= 85:
      print('B+')
  elif score >=80:
      print('B')
  elif score >= 70:
      print('C')
  elif score >= 60:
      print('D')
  else:
      print('E')
  

  

3.2.3 if not …

• as follows:

  a = 3
  b = 9
  c = (a>b)
  
  if not c:
      print('a<=b')
  else:
      print('a>b')
  

  

3.2.4 if … is not …

• In this case, not is usually used together with is, and is not can be understood literally. Understood as if x (is not) ''.

  a = ''
  if a is not '':
      print('a 非空')
  else:
      print('a 为空')
  

  

4. while statement

4.1 Grammar

• as follows:

  while 判断条件(condition)：
      执行语句(statements)……
  

  or

  while <expr>:
  	<statement(s)>
  else:
      <additional_statement(s)>
  

4.2 Example

4.2.1 Example 1

• as follows:

  n = 10
  i = 1
  sum = 0
  while i <= n:
      sum = sum + i
      i += 1
  
      
  print(sum)
  

  

4.2.2 Example 2

• as follows:

  i = 1
  n = 10
  sum = 0
  
  while i <= n:
      sum = sum + i
      i = i+1
  else:
      print("求和结束，此时，sum = ",sum)
  
      
  求和结束，此时，sum =  55
  

  

5. Other

5.1 The difference between import and from...import in python

5.1.1 Theoretical Differences

• import module: Import a module; Note: It is equivalent to importing a folder, which is a relative path.
• from...import: imported into a modulea function; Note: It is equivalent to importing a file in a folder, which is an absolute path.

5.1.2 Differences in usage

• import
  模块.函数, as follows:

  a = decimal.Decimal(3.456)
  

• 
  直接使用函数名Just use from...import , as follows:

  c = Decimal(5.6)
  

5.1.3 Examples

• as follows:

  import decimal
  a = decimal.Decimal(3.456)
  

  

  from decimal import Decimal
  a = Decimal(2.3)
  

  

5.2