Tips for nesting if and else statements in for loops in Python

for...[if]...Construct List (List comprehension)
1. Simple for...[if]... statement
In Python, the for...[if]... statement is a concise way to construct a List. It selects elements that meet the if condition from the list given by for to form a new List, where if can be omitted. of. Here are a few simple examples to illustrate.

>>> a=[12, 3, 4, 6, 7, 13, 21]
>>> newList = [x for x in a]
>>> newList
[12, 3, 4, 6, 7, 13, 21]
>>> newList2 = [x for x in a if x%2==0]
>>> newList2
[12, 4, 6]

With if omitted, newList constructs a List with the same elements as a. However, newList and a are different Lists. Executing b=a, b and newList are different. newList2 is a List composed of elements selected from a that satisfy x%2==0. If you do not use the for...[if].. statement, the following operations are required to build newList2.

>>> newList2=[]
>>> for x in a:
...  if x %2 == 0:
...    newList2.append(x)
>>> newList2
[12, 4, 6]

Obviously, using the for...[if]... statement is more concise.

2. Nested for...[if]... statements
The nested for...[if]... statement can select elements that meet the if condition from multiple Lists to form a new List. Here are a few examples.

>>>a=[12, 3, 4, 6, 7, 13, 21]
>>>b=['a', 'b', 'x']
>>>newList=[(x, y) for x in a for y in b]
>>>newList
[(12, 'a'), (12, 'b'), (12, 'x'), (3, 'a'), (3, 'b'), (3, 'x'), (4, 'a'), (4, 'b'), (4, 'x'), (6, 'a'), (6, 'b'), (6, 'x'), (7, 'a'), (7, 'b'), (7, 'x'), (13, 'a'), (13, 'b'), (13, 'x'), (21, 'a'), (21, 'b'), (21, 'x')]
>>>newList2=[(x, y) for x in a for y in b if x%2==0 and y<'x']
>>>newList2
[(12, 'a'), (12, 'b'), (4, 'a'), (4, 'b'), (6, 'a'), (6, 'b')]

The nested for...[if]... statement is equivalent to multiple for statements. The first for statement is the outermost loop.

Make good use of python’s else clause
In daily coding, the use of branch statements is very common. The logic of code execution is often controlled based on whether certain conditions are met, so everyone will be familiar with if[elif[else]]. The else clause in the branch statement will be executed when other conditions are not met. Appropriate use of branch statements can make our code logic richer.
The use of else clauses in branch statements is basically the same in some common programming languages. It is similar to providing a default execution path and used with conditional judgment statements such as if. Compared with other programming languages ​​​​(c#, java, js etc.) In python, else has some special usages. It can be used with loop statements such as for and while, and can even be used with exception handling try except statements, which can make our code more concise.

1. Use with for/while loop statement
The else clause is immediately followed by the for loop statement. When the loop ends normally (without early exit such as return or break), the logic of the else clause will be executed. Let’s look at an example first:

def print_prime(n):
  for i in xrange(2, n):
    # found = True
    for j in xrange(2, i):
      if i % j == 0:
         # found = False 
        break
    else:
      print "{} it's a prime number".format(i)
    # if found:
         # print "{} it's a prime number".format(i)

print_prime(7)

Result:

2 it's a prime number
3 it's a prime number
5 it's a prime number

A simple example of printing prime numbers. When judging whether a number is a prime number, you need to traverse integers smaller than itself. If any one satisfies divisibility, the judgment ends. Otherwise, print the info that this is a prime number. With else With the blessing, the logic of the entire example is quite "self-expressive", as easy to understand as pseudo code, and compared to setting the flag value when judging divisibility and then judging the flag value at the end of the function to decide whether to print the number when the prime number message , the code is cleaner and there aren't as many "procedural" preparations to describe how to do.
ps: You can run the commented code in the example to compare the effects.

2. Use with try except error control
In exception handling statements, else has similar usage. When the try code block does not throw any exception, the else statement block will be executed.

def my_to_int(str_param):
  try:
    print int(str_param)
  except ValueError:
    print 'cannot convert {} to a integer'.format(str_param)
  else:
    print 'convert {} to integer successfully'.format(str_param)

my_to_int("123")
my_to_int("me123")

Result:

123
convert 123 to integer successfully
cannot convert me123 to a integer

As shown in the print log, when the conversion is successful and no error occurs, the logic in the else statement will be executed. Of course, this example may not have much practical use, but it can roughly illustrate the role of else in error handling. Uses: Simplify logic, avoid using some flag values, and accurately grasp whether an error occurs to perform some actual operations (for example, if an error occurs when saving data, perform a rollback operation in the else statement block, and then immediately Then you can add finally statements to complete some cleaning operations

.

Making good use of the else statement block allows us to write code that is more concise and closer to the semantics of natural language. Of course, it will also be more pythonic. You can slowly understand the subtleties.