Python parallel assignment

Sat, Mar 16, 2013

Python supports parallel assignment like

>>> lang, version = "python", 2.7
>>> print lang, version
python 2.7

values are assigned to each variable without any issues.

>>> x, y, z = 1, 2, x + y
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

First python tries to evaluate x + y expression. Since x, y is defined in same line, python is unable to access the variable x and y, so NameError is raised.

>>> x, y = 1, 2
>>> z, a = x + y, 65
>>> print x, a
1 65

In above code x, y is referenced before so x + y is evaluated and the value is assigned to z.

So don’t assign the values in same line and use it in expression