[Tutor] Python root.

Dave Angel davea at ieee.org
Sun Apr 18 23:08:25 CEST 2010

Aidas wrote:
> <div class="moz-text-flowed" style="font-family: -moz-fixed">Hello.
> In here 
> http://mail.python.org/pipermail/tutor/2001-February/003385.html You 
> had written how to ger root in python. The way is: "from math import 
> sqrtprint sqrt( 49 )".
> I noticed that if I write just "print sqrt(49)" I get nothing. 
I don't get "nothing," I get an error message.  In particular I get:

Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
NameError: name 'sqrt' is not defined

> So why I need to write "from math import sqrt" instead of write just 
> "print sqrt( 49 )"?
> P.S. Sorry about english-I'm lithuanian. :)
As the message says, "sqrt" is not defined in the language.  It's 
included in one of the library modules.  Whenever you need code from an 
external module, whether that module is part of the standard Python 
library or something you wrote, or even a third-party library, you have 
to import it before you can use it.  The default method of importing is:

import math
print math.sqrt(49)

Where the prefix qualifer on sqrt means to run the sqrt() specifically 
from the math module.

When a single function from a particular library module is needed many 
times, it's frequently useful to use the alternate import form:

from math import sqrt

which does two things:

import math
sqrt = math.sqrt

The second line basically gives you an alias, or short name, for the 
function from that module.


More information about the Tutor mailing list