On Tue, Oct 22, 2013 at 07:20:25PM +0000, Key, Gregory E (E S SF RNA FSF 1 C)
wrote:
> I understand that a comma in Python is a separator and not an
> operator. In some of the MatPlotLib examples I see code like this:
>
> line1, = ax1.plot(t, y1, lw=2, color='red', label='1 HZ')
>
> What does the comma do in an assignment statement?
It does "sequence unpacking".
On the right hand side, commas create a tuple:
py> x = 100, 200, 300
py> print(x, type(x))
(100, 200, 300) <class 'tuple'>
On the left hand side, you can think of it as if it creates a tuple of
names, then assigns to each name with the corresponding item from the
other side:
py> a, b, c = 100, 200, 300
py> print(a, b, c)
100 200 300
So this example is conceptually like:
# make a tuple of names
(a, b, c)
# line them up with a sequence on the right hand side
(a, b, c) = (100, 200, 300)
# unpack the sequence on the right and assign item-by-item
a <-- item 0 = 100
b <-- item 1 = 200
c <-- item 2 = 300
So a single comma on the left, like this:
a, = function(args, more_args)
is equivalent to this:
temp <-- function(args, more_args)
a <-- temp[0]
delete temp
except of course the name "temp" isn't literally used.
The values on the right don't have to be a tuple, any sequence will do,
such as strings, lists, or iterators:
py> a, b, c = "XYZ"
py> print(a, b, c)
X Y Z
However, there does have to be the same number of items on both sides:
py> a, b, c = "xy"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: need more than 2 values to unpack
--
Steven
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