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Q&A What is the purpose of `if __name__ == '__main__'`?

It makes difference if the script is being imported. Let's suppose I have a file my_file.py: # my_file.py def some_function(): print('do some stuff') print('calling function:') some_fun...

posted 3y ago by hkotsubo‭ · edited 3y ago by hkotsubo‭

Answer

#3: Post edited by

hkotsubo‭ · 2021-10-21T17:37:27Z (about 3 years ago)

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It makes difference if the script is being imported.
Let's suppose I have a file `my_file.py`:
```python
# my_file.py
def some_function():
print('do some stuff')
print('calling function:')
some_function()
```
If I execute it directly (such as `python my_file.py`), the output is:
```none
calling function:
do some stuff
```
Now suppose that I have another file (`another.py`), that imports `my_file.py` and uses `some_function`:
```python
# another.py
import my_file
print('calling function from my_file')
my_file.some_function()
```
By executing `python another.py`, the output will be:
```none
calling function:
do some stuff
calling function from my_file
do some stuff
```
~~That's because the `import` executes the whole contents of `my_file.py` (the first two lines in the output), and then `another.py` calls the function again.~~
To avoid that, we can change `my_file.py` to this:
```python
# my_file.py modified
def some_function():
print('do some stuff')
if __name__ == "__main__":
print('calling function:')
some_function()
```
If I execute it directly (`python my_file.py`), it still executes the two lines inside the `if` (`print` and `some_function()`).
But if I execute `python another.py`, the output will be:
```none
calling function from my_file
do some stuff
```
Because when `my_file.py` is imported, the code inside that `if` is not executed.
---
That happens because, when a Python source file is executed, some special variables are set, and `__name__` is one of them.
When the file is being executed directly (such as `python my_file.py`), the `__name__` variable is set to `__main__`. But when `my_file.py` is imported, its `__name__` variable is set to `my_file`.
Let's modify the files to see how this works.
`my_file.py`:
```python
print('my_file=', __name__)
```
`another.py`:
```python
import my_file
print('another=', __name__)
```
If I execute `python my_file.py`, the output is:
```none
my_file= __main__
```
But if I execute `python another.py`, the output is:
```none
my_file= my_file
another= __main__
```
Note that, for the file being executed, the `__name__` variable contains the value `__main__`, and for the imported files, the `__name__` variable is the file's name without the `.py` extension.
~~The code above also shows that each module has its own `__name__`. This means that `another.py` could also have the same `if` clause inside it.~~
---
This is useful when a module contains lots of functions/classes that you want to import and use, but it also contains a "main" part that acts as a script by itself (this could be the main program, or unit tests, or a "demo" for the functions, or whatever). This "main" part can be put inside that `if` clause, so you don't run it when the module is imported, as in this case you're interested only in reusing the functions/classes.

It makes difference if the script is being imported.
Let's suppose I have a file `my_file.py`:
```python
# my_file.py
def some_function():
print('do some stuff')
print('calling function:')
some_function()
```
If I execute it directly (such as `python my_file.py`), the output is:
```none
calling function:
do some stuff
```
Now suppose that I have another file (`another.py`), that imports `my_file.py` and uses `some_function`:
```python
# another.py
import my_file
print('calling function from my_file')
my_file.some_function()
```
By executing `python another.py`, the output will be:
```none
calling function:
do some stuff
calling function from my_file
do some stuff
```
That's because the `import` executes the whole contents of `my_file.py` (producing the first two lines in the output), and then `another.py` calls the function again.
To avoid that, we can change `my_file.py` to this:
```python
# my_file.py modified
def some_function():
print('do some stuff')
if __name__ == "__main__":
print('calling function:')
some_function()
```
If I execute it directly (`python my_file.py`), it still executes the two lines inside the `if` (`print` and `some_function()`).
But if I execute `python another.py`, the output will be:
```none
calling function from my_file
do some stuff
```
Because when `my_file.py` is imported, the code inside that `if` is not executed.
---
That happens because, when a Python source file is executed, some special variables are set, and `__name__` is one of them.
When the file is being executed directly (such as `python my_file.py`), the `__name__` variable is set to `__main__`. But when `my_file.py` is imported, its `__name__` variable is set to `my_file`.
Let's modify the files to see how this works.
`my_file.py`:
```python
print('my_file=', __name__)
```
`another.py`:
```python
import my_file
print('another=', __name__)
```
If I execute `python my_file.py`, the output is:
```none
my_file= __main__
```
But if I execute `python another.py`, the output is:
```none
my_file= my_file
another= __main__
```
Note that, for the file being executed, the `__name__` variable contains the value `__main__`, and for the imported files, the `__name__` variable is the file's name without the `.py` extension.
The code above also shows that each module has its own `__name__`. This means that `another.py` could also have the same `if` clause inside it, to check if it's being imported, regardless of other module's `__name__`'s.
---
This is useful when a module contains lots of functions/classes that you want to import and use, but it also contains a "main" part that acts as a script by itself (this could be the main program, or unit tests, or a "demo" for the functions, or whatever). This "main" part can be put inside that `if` clause, so you don't run it when the module is imported, as in this case you're interested only in reusing the functions/classes.

#2: Post edited by

hkotsubo‭ · 2021-10-21T17:29:32Z (about 3 years ago)

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It makes difference if the script is being imported.
Let's suppose I have a file `my_file.py`:
```python
# my_file.py
def some_function():
print('do some stuff')
print('calling function:')
some_function()
```
If I execute it directly (such as `python my_file.py`), the output is:
```none
calling function:
do some stuff
```
Now suppose that I have another file (`another.py`), that imports `my_file.py` and uses `some_function`:
```python
# another.py
import my_file
print('calling function from my_file')
my_file.some_function()
```
By executing `python another.py`, the output will be:
```none
calling function:
do some stuff
calling function from my_file
do some stuff
```
That's because the `import` executes the whole contents of `my_file.py` (the first two lines in the output), and then `another.py` calls the function again.
To avoid that, we can change `my_file.py` to this:
```python
# my_file.py modified
def some_function():
print('do some stuff')
if __name__ == "__main__":
print('calling function:')
some_function()
```
If I execute it directly (`python my_file.py`), it still executes the two lines inside the `if` (`print` and `some_function()`).
But if I execute `python another.py`, the output will be:
```none
calling function from my_file
do some stuff
```
Because when `my_file.py` is imported, the code inside that `if` is not executed.
---
That happens because, when a Python source file is executed, some special variables are set, and `__name__` is one of them.
When the file is being executed directly (such as `python my_file.py`), the `__name__` variable is set to `__main__`. But when `my_file.py` is imported, its `__name__` variable is set to `my_file`.
Let's modify the files to see how this works.
`my_file.py`:
```python
print('my_file=', __name__)
```
`another.py`:
```python
import my_file
print('another=', __name__)
```
If I execute `python my_file.py`, the output is:
```none
my_file= __main__
```
But if I execute `python another.py`, the output is:
```none
my_file= my_file
another= __main__
```
Note that, for the file being executed, the `__name__` variable contains the value `__main__`, and for the imported files, the `__name__` variable is the file's name without the `.py` extension.
---
This is useful when a module contains lots of functions/classes that you want to import and use, but it also contains a "main" part that acts as a script by itself (or unit tests, or a "demo" for the functions, or whatever). This "main" part can be put inside that `if` clause, so you don't run it when the module is imported, as in this case you're interested only in reusing the functions/classes.

It makes difference if the script is being imported.
Let's suppose I have a file `my_file.py`:
```python
# my_file.py
def some_function():
print('do some stuff')
print('calling function:')
some_function()
```
If I execute it directly (such as `python my_file.py`), the output is:
```none
calling function:
do some stuff
```
Now suppose that I have another file (`another.py`), that imports `my_file.py` and uses `some_function`:
```python
# another.py
import my_file
print('calling function from my_file')
my_file.some_function()
```
By executing `python another.py`, the output will be:
```none
calling function:
do some stuff
calling function from my_file
do some stuff
```
That's because the `import` executes the whole contents of `my_file.py` (the first two lines in the output), and then `another.py` calls the function again.
To avoid that, we can change `my_file.py` to this:
```python
# my_file.py modified
def some_function():
print('do some stuff')
if __name__ == "__main__":
print('calling function:')
some_function()
```
If I execute it directly (`python my_file.py`), it still executes the two lines inside the `if` (`print` and `some_function()`).
But if I execute `python another.py`, the output will be:
```none
calling function from my_file
do some stuff
```
Because when `my_file.py` is imported, the code inside that `if` is not executed.
---
That happens because, when a Python source file is executed, some special variables are set, and `__name__` is one of them.
When the file is being executed directly (such as `python my_file.py`), the `__name__` variable is set to `__main__`. But when `my_file.py` is imported, its `__name__` variable is set to `my_file`.
Let's modify the files to see how this works.
`my_file.py`:
```python
print('my_file=', __name__)
```
`another.py`:
```python
import my_file
print('another=', __name__)
```
If I execute `python my_file.py`, the output is:
```none
my_file= __main__
```
But if I execute `python another.py`, the output is:
```none
my_file= my_file
another= __main__
```
Note that, for the file being executed, the `__name__` variable contains the value `__main__`, and for the imported files, the `__name__` variable is the file's name without the `.py` extension.
The code above also shows that each module has its own `__name__`. This means that `another.py` could also have the same `if` clause inside it.
---
This is useful when a module contains lots of functions/classes that you want to import and use, but it also contains a "main" part that acts as a script by itself (this could be the main program, or unit tests, or a "demo" for the functions, or whatever). This "main" part can be put inside that `if` clause, so you don't run it when the module is imported, as in this case you're interested only in reusing the functions/classes.

#1: Initial revision by

hkotsubo‭ · 2021-10-21T17:21:29Z (about 3 years ago)

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It makes difference if the script is being imported.

Let's suppose I have a file `my_file.py`:

```python
# my_file.py
def some_function():
    print('do some stuff')

print('calling function:')
some_function()
```

If I execute it directly (such as `python my_file.py`), the output is:

```none
calling function:
do some stuff
```

Now suppose that I have another file (`another.py`), that imports `my_file.py` and uses `some_function`:

```python
# another.py
import my_file

print('calling function from my_file')
my_file.some_function()
```

By executing `python another.py`, the output will be:

```none
calling function:
do some stuff
calling function from my_file
do some stuff
```

That's because the `import` executes the whole contents of `my_file.py` (the first two lines in the output), and then `another.py` calls the function again.

To avoid that, we can change `my_file.py` to this:

```python
# my_file.py modified
def some_function():
    print('do some stuff')

if __name__ == "__main__":
    print('calling function:')
    some_function()
```

If I execute it directly (`python my_file.py`), it still executes the two lines inside the `if` (`print` and `some_function()`).

But if I execute `python another.py`, the output will be:

```none
calling function from my_file
do some stuff
```

Because when `my_file.py` is imported, the code inside that `if` is not executed.

---
That happens because, when a Python source file is executed, some special variables are set, and `__name__` is one of them.

When the file is being executed directly (such as `python my_file.py`), the `__name__` variable is set to `__main__`. But when `my_file.py` is imported, its `__name__` variable is set to `my_file`.

Let's modify the files to see how this works.

`my_file.py`:

```python
print('my_file=', __name__)
```

`another.py`:

```python
import my_file

print('another=', __name__)
```

If I execute `python my_file.py`, the output is:

```none
my_file= __main__
```

But if I execute `python another.py`, the output is:

```none
my_file= my_file
another= __main__
```

Note that, for the file being executed, the `__name__` variable contains the value `__main__`, and for the imported files, the `__name__` variable is the file's name without the `.py` extension.

---
This is useful when a module contains lots of functions/classes that you want to import and use, but it also contains a "main" part that acts as a script by itself (or unit tests, or a "demo" for the functions, or whatever). This "main" part can be put inside that `if` clause, so you don't run it when the module is imported, as in this case you're interested only in reusing the functions/classes.

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