Summary of usage of else in Python loop statements

Preface

This article discusses Python’s for…else and while…else syntax. These are the most popular syntaxes in Python. One of the least commonly used and most misunderstood features of grammar.

for, while and other loops in Python have an optional else branch (similar to the if statement Like the try statement), it is executed after the loop iteration has completed normally. In other words, if we don't exit the loop any other way than the normal way, the else branch will be executed. That is, there is no break statement, no return statement, or no exception occurs in the loop body.

Let’s take a look at detailed usage examples.

1. Conventional if else usage

##

x = True
if x:
 print 'x is true'
else:
 print 'x is not true'

2. If else shortcut usage

The

if else here can be used as a ternary operator.

mark = 40
is_pass = True if mark >= 50 else False
print "Pass? " + str(is_pass)

3. Use it with the for keyword

when the following conditions are met At this time, the code block under

else will be executed:

1. The statements in the

for loop are executed

2.

for The statements in the loop are not interrupted by the break statement

# 打印 `For loop completed the execution`
for i in range(10):
 print i
else:
 print 'For loop completed the execution'
# 不打印 `For loop completed the execution`
for i in range(10):
 print i
 if i == 5:
 break
else:
 print 'For loop completed the execution'

## 4. Use with the while keyword. Similar to the above, when the following conditions are met, the code block under

else

will be executed: 1.

while

The statements in the loop are executed
2.

while

The statements in the loop are not interrupted by the break statement Break

# 打印 `While loop execution completed`
a = 0
loop = 0
while a <= 10:
 print a
 loop += 1
 a += 1
else:
 print "While loop execution completed"
# 不打印 `While loop execution completed`
a = 50
loop = 0
while a > 10:
 print a
 if loop == 5:
 break
 a += 1
 loop += 1
else:
 print "While loop execution completed"

5. Use and # together with try except ##try except

When used together, if no exception is thrown, the statements in

else can be executed.

file_name = "result.txt"
try:
 f = open(file_name, 'r')
except IOError:
 print 'cannot open', file_name
else:
 # Executes only if file opened properly
 print file_name, 'has', len(f.readlines()), 'lines'
 f.close()

Summary

This is a summary of the usage of else in loop statements in Python That’s basically it. This article still has certain reference value for everyone to learn or use Python. I hope it can be helpful to everyone. If you have any questions, you can leave a message to communicate.

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