__all__ in Python Explained in 15 Seconds

# provided you read pretty fast

Case 1 : No __all__

# a.py

a = 'apple'
b = 'boy'
c = 'cat'
d = 'donkey'
e = 'elephant'
f = 'fish'

# b.py

from a import *

print(dir())

# [..., 'a', 'b', 'c', 'd', 'e', 'f']

^ here, from a import * imports everything from a.py into b.py. Which is why a, b, c, d, e & f show up in b.py's dir()

Case 2: __all__ is defined

# a.py

a = 'apple'
b = 'boy'
c = 'cat'
d = 'donkey'
e = 'elephant'
f = 'fish'

__all__ = ['a', 'b']

# b.py

from a import *

print(dir())

# [..., 'a', 'b']

^ here, from a import * imports only a and b. And this is because of __all__ in a.py telling b.py that it can only import a and b

Note — this works only from from a import * — you can still from a import c, d, e, f and it will work

Why/when to use

If a.py contains a whole bunch of random stuff, but we only need to ‘export’ a and b, we should probably use __all__ so that we don’t accidentally import a whole bunch of clutter when using from a import *

Conclusion

Hope this was clear and easy to understand.

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