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

View Slide

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

View Slide

Modules
Any p
y
t
h
o
n source ﬁle would be counted as a
module.
You import a module to execute and access its
classes, func on deﬁni ons, a ributes.
>
>
> i
m
p
o
r
t o
s
>
>
> o
s
.
p
a
t
h
.
a
b
s
p
a
t
h
(
'
.
'
)
'
/
h
o
m
e
/
t
a
s
d
i
k
/
D
r
o
p
b
o
x
/
t
a
l
k
s
/
c
h
e
n
n
a
i
p
y
/
o
c
t
o
b
e
r
/
s
a
m
p
l
e
c
o
d
e
'
>
>
>
p
o
s
i
x
p
a
t
h would be the module name where the
method a
b
s
p
a
t
h
(
) resides. p
o
s
i
x
p
a
t
h being the
alias name for o
s
.
p
a
t
h in linux systems. 3

View Slide

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
p
r
i
n
t
(
) statement ), then they get executed
when the module is being imported.
# '
s
a
m
p
l
e
c
o
d
e
/
b
a
s
i
c
p
a
c
k
a
g
e
/
'
>
>
> i
m
p
o
r
t b
a
s
i
c
p
a
c
k
a
g
e
.
b
a
r
i
n
s
i
d
e b
a
s
i
c
p
a
c
k
a
g
e
/
_
_
i
n
i
t
_
_
.
p
y
i
n
s
i
d
e '
b
a
s
i
c
p
a
c
k
a
g
e
/
b
a
r
'
>
>
>
4

View Slide

Packages
Used to orgranize larger number of modules in a
systema c manner.
One way of accessing individual modules is using
the i
m
p
o
r
t f
o
o
.
b
a
r import style.
# t
y
p
i
c
a
l p
a
c
k
a
g
e s
t
r
u
c
t
u
r
i
n
g
$ t
r
e
e b
a
s
i
c
p
a
c
k
a
g
e
/
b
a
s
i
c
p
a
c
k
a
g
e
├
─
─ b
a
r
.
p
y
├
─
─ f
o
o
.
p
y
└
─
─ _
_
i
n
i
t
_
_
.
p
y
5

View Slide

Why packages?
b
u
m
b
l
e
b
e
e
├
─
─ c
o
n
s
t
a
n
t
s
.
p
y
├
─
─ c
o
r
e
.
p
y
├
─
─ e
x
c
e
p
t
i
o
n
s
.
p
y
├
─
─ h
e
l
p
e
r
s
│ ├
─
─ _
_
i
n
i
t
_
_
.
p
y
│ └
─
─ v
w
o
_
h
e
l
p
e
r
s
.
p
y
├
─
─ _
_
i
n
i
t
_
_
.
p
y
└
─
─ v
w
o
├
─
─ _
_
i
n
i
t
_
_
.
p
y
└
─
─ s
m
a
r
t
_
c
o
d
e
.
p
y
Looks good?
6

View Slide

Diﬀerent styles for impor ng
modules
7

View Slide

from module import foo
This essen ally imports the module first then
picks up specific parts from the module to be
available locally.
>
>
> f
r
o
m b
a
s
i
c
p
a
c
k
a
g
e i
m
p
o
r
t f
o
o
i
n
s
i
d
e b
a
s
i
c
p
a
c
k
a
g
e
/
_
_
i
n
i
t
_
_
.
p
y
i
n
s
i
d
e '
b
a
s
i
c
p
a
c
k
a
g
e
/
f
o
o
.
p
y
' w
i
t
h a v
a
r
i
a
b
l
e i
n i
t
>
>
>
allows using the parts of the module without
giving the full prefix before it.
8

View Slide

from module import *
Brings out all the symbols from the module and
makes them available in the namespace.
>
>
> f
r
o
m b
a
s
i
c
p
a
c
k
a
g
e
_
a
l
l i
m
p
o
r
t *
i
n
s
i
d
e b
a
s
i
c
p
a
c
k
a
g
e
_
a
l
l
/
_
_
i
n
i
t
_
_
.
p
y
i
n
s
i
d
e '
b
a
s
i
c
p
a
c
k
a
g
e
_
a
l
l
/
f
o
o
.
p
y
' w
i
t
h a v
a
r
i
a
b
l
e i
n i
t
i
n
s
i
d
e '
b
a
s
i
c
p
a
c
k
a
g
e
_
a
l
l
/
b
a
r
.
p
y
'
>
>
>
You can use _
_
a
l
l
_
_ inside your _
_
i
n
i
t
_
_
.
p
y
module to import the modules which you need.
Generally not a good idea! Namespace collisions
can occur.
9

View Slide

Takeaways so far
The way you import a module doesn't actually
change the working of the module.
Difference between
i
m
p
o
r
t f
o
o
.
b
a
r and f
r
o
m f
o
o i
m
p
o
r
t b
a
r ?
the difference is subjec ve. Pick one style and
be consistent with it.
doing a f
r
o
m f
o
o i
m
p
o
r
t b
a
r is more efficient.
p
y
t
h
o
n imports the whole file! period.
10

View Slide

Module names
naming modules follow the general variable
naming conven on.
# B
a
d c
h
o
i
c
e
s
$ t
o
u
c
h 2
f
o
o
.
p
y M
y
A
w
e
s
o
m
e
F
o
o
.
p
y o
s
.
p
y
# G
o
o
d c
h
o
i
c
e
s
$ t
o
u
c
h f
o
o
.
p
y a
_
l
a
r
g
e
_
m
o
d
u
l
e
_
n
a
m
e
.
p
y
Don't use Non‐ASCII characters while doing so.
Avoid crea ng module names which conﬂict with
the standard library modules.
11

View Slide

Module lookup
If it's not in the python path, it just won't import.
>
>
> p
p
r
i
n
t
(
s
y
s
.
p
a
t
h
)
[
'
'
,
'
/
u
s
r
/
l
i
b
/
p
y
t
h
o
n
3
5
.
z
i
p
'
,
.
.
.
'
/
u
s
r
/
l
i
b
/
p
y
t
h
o
n
3
/
d
i
s
t

p
a
c
k
a
g
e
s
'
]
Explicitly bring a module inside your path
>
>
> i
m
p
o
r
t s
y
s
>
>
> s
y
s
.
p
a
t
h
.
a
p
p
e
n
d
(
'
/
a
b
s
o
u
l
e
/
p
a
t
h
/
t
o
/
m
o
d
u
l
e
'
)
12

View Slide

Modules get imported Only once!
13

View Slide

But I really want to import it again!
>
>
> f
r
o
m i
m
p
o
r
t
l
i
b i
m
p
o
r
t r
e
l
o
a
d
>
>
> r
e
l
o
a
d
(
f
o
o
)
This is generally not recommended!
If you do so, zombies will spawn.
No really!
14

View Slide

Implicit Rela ve imports
$ r
o
d
/
f
o
o
.
p
y
b
a
r
.
p
y
_
_
i
n
i
t
_
_
.
p
y
So want to have some things from f
o
o
.
p
y inside
b
a
r
.
p
y ? Nothing uncommon.
# p
y
t
h
o
n 2
# i
n
s
i
d
e "
b
a
r
.
p
y
"
i
m
p
o
r
t f
o
o
Don't do it! Works in p
y
t
h
o
n
2 but doesn't work in
p
y
t
h
o
n
3 15

View Slide

How do I ﬁx it?
16

View Slide

Absolute rela ve imports
One way to ﬁx it would be using the name of it's
top level package name r
e
l
a
t
i
v
e
i
m
p
o
r
t
s.
# r
e
l
a
t
i
v
e
i
m
p
o
r
t
s
/
f
o
o
.
p
y
f
r
o
m r
e
l
a
t
i
v
e
i
m
p
o
r
t
s i
m
p
o
r
t b
a
r
This works, but is bri le!
What if you wanted to change the name of the
top level package?
Errors!!!!
17

View Slide

Explicit rela ve imports
A be er way would be to
# e
x
p
l
i
c
i
t
i
m
p
o
r
t
s
/
b
a
r
.
p
y
f
r
o
m . i
m
p
o
r
t f
o
o
Works even when you rename the root level
package for whatever the reason may be
(eg: you renamed it to e
x
p
l
i
c
i
t
i
m
p
o
r
t
s
_
v
1
_
0 )
$ m
v e
x
p
l
i
c
i
t
i
m
p
o
r
t
s
/ n
e
w
i
m
p
o
r
t
s
/
18

View Slide

The leading (.) would be used to move up a
directory.
# l
o
o
k f
o
r f
o
o
.
p
y i
n t
h
e s
a
m
e l
e
v
e
l
f
r
o
m . i
m
p
o
r
t f
o
o
# g
o a d
i
r u
p a
n
d i
m
p
o
r
t f
o
o
.
p
y
f
r
o
m .
. i
m
p
o
r
t f
o
o
# g
o a d
i
r u
p a
n
d e
n
t
e
r p
l
i
n
o
/ a
n
d l
o
o
k f
o
r b
a
r
.
p
y
f
r
o
m .
.
p
l
i
n
o i
m
p
o
r
t b
a
r
19

View Slide

_
_
i
n
i
t
_
_
.
p
y
20

View Slide

What should you put into it?
Most of the me, it's empty!
S ching together submodules:
# m
i
n
i
o
n
s
/
f
o
o
.
p
y
c
l
a
s
s F
o
o
(
o
b
j
e
c
t
)
:
p
a
s
s
# m
i
n
i
o
n
s
/
b
a
r
.
p
y
c
l
a
s
s B
a
r
(
o
b
j
e
c
t
)
:
p
a
s
s
# m
i
n
i
o
n
s
/
_
_
i
n
i
t
_
_
.
p
y
f
r
o
m .
f
o
o i
m
p
o
r
t F
o
o
f
r
o
m .
b
a
r i
m
p
o
r
t B
a
r
21

View Slide

Advantage?
Headache free imports for small modules
>
>
> i
m
p
o
r
t m
i
n
i
o
n
s
i
n
s
i
d
e m
i
n
i
o
n
s
/
_
_
i
n
i
t
_
_
.
p
y
i
n
s
i
d
e '
m
i
n
i
o
n
s
/
f
o
o
.
p
y
' w
i
t
h a v
a
r
i
a
b
l
e i
n i
t
i
n
s
i
d
e '
m
i
n
i
o
n
s
/
b
a
r
.
p
y
'
>
>
> a = m
i
n
i
o
n
s
.
F
o
o
(
)
>
>
> b = m
i
n
i
o
n
s
.
B
a
r
(
)
controlling import behaviour of f
r
o
m f
o
o i
m
p
o
r
t *
using the _
_
a
l
l
_
_ variable inside _
_
i
n
i
t
_
_
.
p
y
22

View Slide

Performance anybody?
Should I put the whole python package inside the
_
_
i
n
i
t
_
_
.
p
y?
Yes. If it's small!
Not a good idea if you have a very large project!
23

View Slide

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

View Slide

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

View Slide

Demystifying how imports work in Python

Demystifying how imports work in Python

Tasdik Rahman

More Decks by Tasdik Rahman

Other Decks in Programming

Featured

Transcript