stay Python In the related code , We often encounter the following code snippet ：
# stuff
if __name__ == "__main__":
# do stuff
This article will try to use as simple examples as possible to explain what happened here , And the need to use if __name__=="__main__" The circumstances of . Please note that , In the above code name and main Before and after 2 Underscore characters .
Gossip , Let's start straight ！
When we run our Python Script time , The variables in this script __name__ The value of is usually __main__. Let's look at an example ：
# first.py
print(__name__)
We run the above script in the terminal , The code is as follows ：
python first.py
Output is as follows ：
__main__
Observe the above output , When we run the script first.py When , Variable __name__ The value of is set to __main__.
The above example is relatively simple , It's also relatively easy to understand . Next, let's take an example with multiple files , Suppose we have three scripts , Respectively a.py,b.py as well as c.py.
among ,a.py Is as follows ：
# a.py
print("__name__ in a.py:", __name__)
from b import *
from c import *
here ,b.py Is as follows :
# b.py
print("__name__ in b.py:", __name__)
c.py Is as follows :
# c.py
print("__name__ in c.py:", __name__)
If we run the command python a.py The results are as follows ：
__name__ in a.py: __main__
__name__ in b.py: b
__name__ in c.py: c
Notice in the script a.py We see __name__ The value of is __main__, At this point in the script b.py in __name__ The value of is b, meanwhile c.py in __name__ The value of is c. This is because the script we run is a.py, Then the script is a.py Medium variable __name__ The value of will be set to string __main__.
Besides , Script b.py and c.py The variables in the ·__name__ Keep as b and c, This is because the script b.py and c.py Not a direct script . It's the script we run a.py Called b.py and c.py Medium print function .
If we run alone python b.py The results are as follows ：
__name__ in b.py: __main__
Accordingly, we run python c.py, The results are as follows ：
__name__ in c.py: __main__
Observe the above two outputs , When we run the script directly b.py or c.py, The variables in the corresponding script __name__ Will be set to __main__.
As Python developer , We usually create multiple projects in one project Python File instead of writing all the code in one Python In file . So we usually have a master Python The file is used as the entry of the project , At the same time, other files contain some auxiliary function implementations .
Let's look at an example , Suppose we have two Python file , as follows ：
# main.py
from helper import *
print(greet("bob"))
Auxiliary function implementation helper.py The contents are as follows ：
# helper.py
def greet(name):
return "hello " + name
print(greet("testname"))
If we run the command at this time python main.py, The results are as follows ：
hello bob
hello testname
Mainly the above import Statement will be from helper.py Introduce all statements in , therefore helper.py Test statements in print(greet("testname")) Will also be implemented . Although we can do this by adding comments or masking comments main.py or helper.py To control the output of the test statement , But this situation is the statement if __name__=="__main__" The situation of showing one's skill .
In view of the above situation , Code rectification , as follows ：
# main.py
from helper import *
print(greet("bob"))
We modify the file helper.py The contents are as follows ：
def greet(name):
return "hello " + name
if __name__ == "__main__":
print(greet("testname"))
here , We carry out orders python main.py, The results are as follows ：
hello bob
Be careful , here helper.py The variables in the __name__ The value of is helper, therefore helper.py Medium if The statement doesn't hold , In turn, our test statements will not be executed print(greet("testname")).
here , If we run the command alone python helper.py , The results are as follows ：
hello testname
Separate operation helper.py when , At this point, change the variables in the script __name__ Will be set to __main__, The test will be executed at this time , Output the corresponding test results .
This article focuses on the introduction Python Common in if __name__=="__main__" , It focuses on the analysis of the principle behind it and the application scenario .
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