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Let's talk about the four ways to import something from a module in Python.
The first way is to use the import
statement, using the syntax import module_name
:
>>> import math
We've just imported the math
module.
We can use the things within this module by looking up attributes on the math
module object.
For example, we can get the square root of a number by calling math.sqrt
:
>>> math.sqrt(25)
5.0
Or we can access the number π with math.pi
:
>>> math.pi
3.141592653589793
But we can't access just pi
or sqrt
(without that math.
prefix):
>>> pi
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'pi' is not defined
>>> sqrt(25)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'sqrt' is not defined
>>>
The reason is, when we import a module using that import module_name
syntax, we get just one thing: a variable that points to a module object.
>>> import math
>>> math
<module 'math' (built-in)>
Everything in that module is accessible as an attribute on that module object:
>>> math.pi
3.141592653589793
If you wanted to just type pi
instead of math.pi
, you could use the from
syntax for importing:
>>> from math import pi
>>> pi
3.141592653589793
So now we have pi
, but not sqrt
(because we didn't import it):
>>> sqrt(25)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'sqrt' is not defined
And we don't have access to the whole math
module:
>>> math
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'math' is not defined
If you want to import multiple things from a module using this from
syntax, you could put commas each thing you'd like to import:
>>> from math import pi, sqrt
>>> pi
3.141592653589793
>>> sqrt(25)
5.0
That from math import pi, sqrt
line plucks out pi
and sqrt
from the math
module, but nothing else.
If you use the from
syntax to grab specific things from a module and you grab something of the same name from two different modules, you're in for trouble.
Here we're importing sqrt
from the math
module and sqrt
from the cmath
module (cmath
is for complex numbers):
>>> from math import sqrt
>>> from cmath import sqrt
The sqrt
function we end up with came from the cmath
module:
>>> sqrt(25)
(5+0j)
We imported two different functions named sqrt
and the last one won.
We first got sqrt
from the math
module and then we overwrote our local sqrt
variable with the sqrt
function from the cmath
module.
To fix this we could rename sqrt
from the cmath
module as we import it, by using the as
syntax:
>>> from math import sqrt
>>> from cmath import sqrt as csqrt
So, sqrt
is now the sqrt
function from the math
module and csqrt
is the one from the cmath
module:
>>> sqrt(25)
5.0
>>> csqrt(25)
(5+0j)
If you look at the function the csqrt
variable points to, you'll even see that it's actually called sqrt
:
>>> csqrt
<built-in function sqrt>
We pointed our local variable csqrt
to the sqrt
function within the cmath
module.
You can also use the as
syntax when importing a whole module.
>>> import math as m
The variable m
now points to the math
module object which means we can't say math.pi
anymore:
>>> math.pi
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'math' is not defined
Instead we can use m.pi
or m.sqrt
:
>>> m.pi
3.141592653589793
>>> m.sqrt(25)
5.0
You likely won't see this convention used often except in specific Python communities where it's commonplace.
For example, in the Pandas world people often import pandas
as pd
:
>> import pandas as pd
And in the NumPy world, people often import numpy
as np
:
import numpy as np
It's not very common to see module names shortened using the as
syntax, except in areas where it's conventional (like Pandas and NumPy).
Trey says: If I saw m.pi
in your code, I'd probably find it a little bit odd.
So there's four different ways to import:
import random
from random import choice, randint
import pandas as pd
from os.path import join as join_path
That last one is usually done to avoid a name collision or sometimes to make a more descriptive name (though that's not very common).
Use from
for short and descriptive variable names
I tend to use the from
syntax most (number 2 above) because I prefer short and descriptive variable names.
Import the whole module if needed to avoid ambiguity.
If there's a name like choice
that isn't as clear as random.choice
, then I prefer to import the whole module for a more descriptive name
Avoid renaming imports.
I very rarely use the as
syntax (unless I'm in the pandas
or numpy
worlds, where it's common convention).
And I almost never use the as
syntax with from
unless I'm avoiding a name collision.
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Modules are the tool we use for breaking up our code into multiple files in Python.
When you write a .py
file, you're making a Python module.
You can import your own modules, modules included in the Python standard library, or modules in third-party packages.
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