What does the comma in this assignment statement do?

Question

I was looking through an interesting example script I found (at this site, last example line 124), and I\'m struggling to understand what the comma after particles

Answer 1

Have a look at what plot call returns. In your case it's a list with one element:

>>> import matplotlib.pyplot as plt
>>> ax = plt.gca()
>>> ax.plot([], [], 'bo', ms=6)
[]

Now it would more useful in this case to have a handle on the actual Line2D object, using unpacking with h, = ax.plot(...) rather than a spurious container around it as you would get with

h = ax.plot([], [], 'bo', ms=6)

The latter would require you an extra step later on e.g.

h[0].set_data(...)

Return value of plot is always a list, because sometimes it needs to return more than one line object. It makes more sense to return even single lines inside a list, so that client code doesn't have to handle each case in a different way.

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The reason your unpacking a, = [2,3] fails is that there are 2 things to unpack on the right side, and only one variable. You would need to use a,b = [2,3] to unpack that.