line, = plot(x,sin(x)) what does comma stand for?

Question

I'm trying to make an animated plot. Here is an example code:

from pylab import *
import time

ion()

tstart = time.time()               # for profiling
x = arange(0,2*pi,0.01)            # x-array
line, = plot(x,sin(x))
for i in arange(1,200):
    line.set_ydata(sin(x+i/10.0))  # update the data
    draw()                         # redraw the canvas

print 'FPS:' , 200/(time.time()-tstart)

I don't understand the line,. Without comma, the code doesn't work.

possible duplicate of [An unusual Python syntax element frequently used in Matplotlib](http://stackoverflow.com/questions/9731779/an-unusual-python-syntax-element-frequently-used-in-matplotlib) — Ignacio Vazquez-Abrams, May 02 '12 at 22:27

Fred Foo · Accepted Answer · 2012-11-27T21:44:43.067

17

The comma is Python syntax that denotes either a single-element tuple. E.g.,

>>> tuple([1])
(1,)

In this case, it is used for argument unpacking: plot returns a single-element list, which is unpacked into line:

>>> x, y = [1, 2]
>>> x
1
>>> y
2
>>> z, = [3]
>>> z
3

An alternative, perhaps more readable way of doing this is to use list-like syntax:

>>> [z] = [4]
>>> z
4

though the z, = is more common in Python code.

edited Nov 27 '12 at 21:44

answered May 02 '12 at 22:22

Fred Foo

355,277
75
744
836

6

Just outsped me, +1, but I disagree that a more readable way of doing this is using list-like syntax, I would use this instead: `line = plot(x, sin(x))[0]`. – orlp May 02 '12 at 22:23
Or even tuple-like syntax: `(z) = [4]` :) – ovgolovin May 02 '12 at 22:25
2

@nightcracker: Indexing with `[0]` isn't equivalent to unpacking, since it doesn't check whether the number of elements in the return value of `plot` matches the number of variables being assigned to. – Fred Foo May 02 '12 at 22:26
Raimond Hettinger about unpacking: https://twitter.com/#!/raymondh/status/190568767158358017 – ovgolovin May 02 '12 at 22:26
2

@ovgolovin: `(z) = [4]` sets `z` to the list `[4]`. – Fred Foo May 02 '12 at 22:27
But you can do `(z,) = [4]`, so that `z` is set to the number `4` – Francesco Pasa Apr 27 '14 at 11:05

score 0 · Answer 2 · answered Oct 25 '18 at 02:47

0

case1:

a=1,
type(a)
tuple

case2:

a=1
type(a)
int

answered Oct 25 '18 at 02:47

Minux

51
1
8

line, = plot(x,sin(x)) what does comma stand for?

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