Demystifying how imports work in Python

Materials @ h ps://github.com/prodicus/talks
Demys fying how imports
work in Python
Tasdik Rahman (@tasdikrahman)
Presented @ ChennaiPy, October'16 meetup

Requirements
Python 3.4 or newer.
No extra 3rd party extensions needed.
Coming over for this meetup!
2

Modules
Any python source ﬁle would be counted as a
module.
You import a module to execute and access its
classes, func on deﬁni ons, a ributes.
>>> import os
>>> os.path.abspath('.')
'/home/tasdik/Dropbox/talks/chennaipy/october/samplecode'
>>>
posixpath would be the module name where the
method abspath() resides. posixpath being the
alias name for os.path in linux systems. 3

What happens when you import a module?
It being a python script, the statements start
ge ng executed from top to bo om of the
source ﬁle.
If there are any tasks in the statements ( eg: a
print() statement ), then they get executed
when the module is being imported.
# 'samplecode/basicpackage/'
>>> import basicpackage.bar
inside basicpackage/__init__.py
inside 'basicpackage/bar'
>>>
4

Packages
Used to orgranize larger number of modules in a
systema c manner.
One way of accessing individual modules is using
the import foo.bar import style.
# typical package structuring
$ tree basicpackage/
basicpackage
├── bar.py
├── foo.py
└── __init__.py
5

Why packages?
bumblebee
├── constants.py
├── core.py
├── exceptions.py
├── helpers
│   ├── __init__.py
│   └── vwo_helpers.py
├── __init__.py
└── vwo
    ├── __init__.py
    └── smart_code.py
Looks good?
6

Diﬀerent styles for impor ng
modules
7

from module import foo
This essen ally imports the module first then
picks up specific parts from the module to be
available locally.
>>> from basicpackage import foo
inside basicpackage/__init__.py
inside 'basicpackage/foo.py' with a variable in it
>>>
allows using the parts of the module without
giving the full prefix before it.
8

from module import *
Brings out all the symbols from the module and
makes them available in the namespace.
>>> from basicpackage_all import *
inside basicpackage_all/__init__.py
inside 'basicpackage_all/foo.py' with a variable in it
inside 'basicpackage_all/bar.py'
>>>
You can use __all__ inside your __init__.py
module to import the modules which you need.
Generally not a good idea! Namespace collisions
can occur.
9

Takeaways so far
The way you import a module doesn't actually
change the working of the module.
Difference between
import foo.bar and from foo import bar ?
the difference is subjec ve. Pick one style and
be consistent with it.
doing a from foo import bar is more efficient.
python imports the whole file! period.
10

Module names
naming modules follow the general variable
naming conven on.
# Bad choices
$ touch 2foo.py MyAwesomeFoo.py os.py

# Good choices
$ touch foo.py a_large_module_name.py
Don't use Non‐ASCII characters while doing so.
Avoid crea ng module names which conﬂict with
the standard library modules.
11

Module lookup
If it's not in the python path, it just won't import.
>>> pprint(sys.path)
['',
'/usr/lib/python35.zip',
...
'/usr/lib/python3/distpackages']

Explicitly bring a module inside your path
>>> import sys
>>> sys.path.append('/absoule/path/to/module')
12

Modules get imported Only once!
13

But I really want to import it again!
>>> from importlib import reload
>>> reload(foo)
This is generally not recommended!
If you do so, zombies will spawn.
No really!
14

Implicit Rela ve imports
$ rod/
    foo.py
    bar.py
    __init__.py
So want to have some things from foo.py inside
bar.py ? Nothing uncommon.
# python 2
# inside "bar.py"
import foo
Don't do it! Works in python2 but doesn't work in
python3 15

Absolute rela ve imports
One way to ﬁx it would be using the name of it's
top level package name relativeimports.
# relativeimports/foo.py
from relativeimports import bar

This works, but is bri le!
What if you wanted to change the name of the
top level package?
Errors!!!!
17

Explicit rela ve imports
A be er way would be to
# explicitimports/bar.py
from . import foo
Works even when you rename the root level
package for whatever the reason may be
(eg: you renamed it to explicitimports_v1_0 )
$ mv explicitimports/ newimports/
18

The leading (.) would be used to move up a
directory.
# look for foo.py in the same level
from . import foo

# go a dir up and import foo.py
from .. import foo

# go a dir up and enter plino/ and look for bar.py
from ..plino import bar
19

What should you put into it?
Most of the me, it's empty!
S ching together submodules:
# minions/foo.py
class Foo(object):
pass

# minions/bar.py
class Bar(object):
pass

# minions/__init__.py
from .foo import Foo
from .bar import Bar

21

Advantage?
Headache free imports for small modules
>>> import minions
inside minions/__init__.py
inside 'minions/foo.py' with a variable in it
inside 'minions/bar.py'
>>> a = minions.Foo()
>>> b = minions.Bar()
controlling import behaviour of from foo import *
using the __all__ variable inside __init__.py
22

Performance anybody?
Should I put the whole python package inside the
__init__.py?
Yes. If it's small!
Not a good idea if you have a very large project!
23

References
h ps://docs.python.org/3/tutorial/modules.html
h ps://docs.python.org/3/reference/import.html
h ps://docs.python.org/3/reference/execu onm
odel.html
h ps://docs.python.org/3/library/distribu on.ht
ml
24

Ques ons?
Would be happy to answer them!
h p://tasdikrahman.me/
Twi er (@tasdikrahman)
Github (@prodicus)
25

Demystifying how imports work in Python

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