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I have always been using this method:

from sys import argv

and use argv with just argv. But there is a convention of using this:

import sys

and using the argv by sys.argv

The second method makes the code self documented and I (really) adhere to it. But the reason I prefer first method is it is fast because we are importing only the function that is needed rather than import the whole module (which contains more useless functions which python will waste time importing them). Note that I need just argv and all other functions from sys are useless to me.

So my questions are. Does the first method really makes the script fast? Which method is preferred most? Why?

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4 Answers

Importing the module doesn't waste anything; the module is always fully imported (into the sys.modules mapping), so wether you use import sys or from sys import argv makes no odds.

The only difference between the two statements is what name is bound; import sys binds the name sys to the module (so sys -> sys.modules['sys']), while from sys import argv binds a different name, argv, pointing straight at the attribute contained inside of the module (so argv -> sys.modules['sys'].argv). The rest of the sys module is still there, whether you use anything else from the module or not.

There is also no performance difference between the two approaches. Yes, sys.argv has to look up two things; it has to look up sys in your global namespace (finds the module), then look up the attribute argv. And yes, by using from sys import argv you can skip the attribute lookup, since you already have a direct reference to the attribute. But the import statement still has to do that work, it looks up the same attribute when importing, and you'll only ever need to use argv once. If you had to use argv thousands of times in a loop it could perhaps make a difference, but in this specific case it really does not.

The choice between one or the other then, should be based on coding style instead.

In a large module, I'd certainly use import sys; code documentation matters, and using sys.argv somewhere in a large module makes it much clearer what you are referring to than just argv ever would.

If the only place you use argv is in a '__main__' block to call a main() function, by all means use from sys import argv if you feel happier about that:

if __name__ == '__main__':
    from sys import argv

I'd still use import sys there myself. All things being equal (and they are, exactly, in terms of performance and number of characters used to write it), that is just easier on the eye for me.

If you are importing something else altogether, then perhaps performance comes into play. But only if you use a specific name in a module many times over, in a critical loop for example. But then creating a local name (within a function) is going to be faster still:

 import somemodule

 def somefunction():
      localname = somemodule.somefunctionorother
      while test:
          # huge, critical loop
          foo = localname(bar)
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There is also the situation where you have a package with subpackages or modules that exposes an attribute of one of those subpackages/modules in the top level package. Using from...import allows you to do package.attribute rather than package.subpackage_or_module.attribute, which can be useful if you have logical or conceptual groupings within the package but want to make things a bit more convenient for users of your package. (numpy does something like this, I believe.) –  JAB Jan 30 at 16:59
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There two reasons in favor of import module, rather than from module import function.

First is the namespace. Importing function into global namespace you risk name clashes.

Second isn't that relevant to standard modules, but significant for you own ones. Especially during development. It's the option to reload() a module. Consider this:

from module import func
# func still points to the old code

On the other hand

import module
# module.func points to the new code

As for speed...

we are importing only the function that is needed rather than import the whole module (which contains more useless functions which python will waste time importing them)

Whether you import a module or import a function from a module, Python will parse whole module. So either way, module is imported. "Importing a function" is nothing more than binding to a name. In fact import module is less work for interpreter, than from module import func.

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I use from imports whenever it improves readability. For example, I prefer (semicolons are only to save space here):

from collections import defaultdict
from foomodule import FooBar, FooBaz
from twisted.internet.protocol import Factory
defaultdict(); FooBar(); FooBaz(); Factory()

instead of:

import collections
import foomodule
import twisted.internet.protocol
collections.defaultdict(); foomodule.FooBar(); foomodule.FooBaz()

The latter is harder to read (and write) for me because it contains so much redundant information. Also, it's useful to know ahead of time what parts of a module I'm using.

I prefer regular imports if I'm using lots of short names from a module:

import sys
sys.argv; sys.stderr; sys.exit()

Or if a name is so generic that it doesn't make sense outside of its namespace:

import json

from json import loads
loads(foo)  # potentially confusing
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Although import sys and from sys import agrv both import the entire sys module, the latter uses name binding so only the argv module is accessible to rest of the code.

For some people this would be the preferred style since it only makes accessible the function you explicitly stated.

It does however introduce potential name conflicts. What if you had another module named argv? Note you can also explicitly import the function and rename with from sys import argv as sys_argv, a convention that meets the explicit import and is less likely to gave name space collisions.

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