Why is “import *” bad?

Question

It is recommended to not to use import * in Python.

Can anyone please share the reason for that, so that I can avoid it doing next time?

Answer 1

These are all good answers. I'm going to add that when teaching new people to code in Python, dealing with import * is very difficult. Even if you or they didn't write the code, it's still a stumbling block.

I teach children (about 8 years old) to program in Python to manipulate Minecraft. I like to give them a helpful coding environment to work with (Atom Editor) and teach REPL-driven development (via bpython). In Atom I find that the hints/completion works just as effectively as bpython. Luckily, unlike some other statistical analysis tools, Atom is not fooled by import *.

However, lets take this example... In this wrapper they from local_module import * a bunch modules including this list of blocks. Let's ignore the risk of namespace collisions. By doing from mcpi.block import * they make this entire list of obscure types of blocks something that you have to go look at to know what is available. If they had instead used from mcpi import block, then you could type walls = block. and then an autocomplete list would pop up.

Answer 2

According to the Zen of Python:

Explicit is better than implicit.

... can't argue with that, surely?

Answer 3

http://docs.python.org/tutorial/modules.html

Note that in general the practice of importing * from a module or package is frowned upon, since it often causes poorly readable code.

Answer 4

• Because it puts a lot of stuff into your namespace (might shadow some other object from previous import and you won't know about it).

• Because you don't know exactly what is imported and can't easily find from which module a certain thing was imported (readability).

• Because you can't use cool tools like pyflakes to statically detect errors in your code.

Answer 5

It is a very BAD practice for two reasons:

1. Code Readability
2. Risk of overriding the variables/functions etc

---

For point 1: Let's see an example of this:

from module1 import *
from module2 import *
from module3 import *

a = b + c - d

Here, on seeing the code no one will get idea regarding from which module b, c and d actually belongs.

On the other way, if you do it like:

#                   v  v  will know that these are from module1
from module1 import b, c   # way 1
import module2             # way 2

a = b + c - module2.d
#            ^ will know it is from module2

It is much cleaner for you, and also the new person joining your team will have better idea.

For point 2: Let say both module1 and module2 have variable as b. When I do:

from module1 import *
from module2 import *

print b  # will print the value from module2

Here the value from module1 is lost. It will be hard to debug why the code is not working even if b is declared in module1 and I have written the code expecting my code to use module1.b

If you have same variables in different modules, and you do not want to import entire module, you may even do:

from module1 import b as mod1b
from module2 import b as mod2b