CODEWITHSUNDEEP
pythonmatplotlib
physicsGuy
physicsGuy

Reputation: 3675

Matplotlib - Default optional arguments for plot

I came across python behavior that I would like to understand. Take a look at the following lines.

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0,1,100)
plt.plot(x, x, 'k', color=None)
plt.plot(x, x**2, 'k', color=None)

According to the matplotlib documentation the plot-kwargs are defined in the Line2D class. There, we find that color takes 'None' as default argument. Hence the plot ought to be black and white. However, the output is colorful.

enter image description here

If I remove the optional color argument, I get black lines. So my question is, why is the above plot colored, even though I passed None as a color-option?

Edit: I understand that removing color=None leads to 'k' being used as the color option. However, I would like to understand how matplotlib internally can distuingish between 

plt.plot(x, x, 'k', color=None)

and

plt.plot(x, x, 'k')

if the color argument is None by default. (Which it is according to the documentation.)

Upvotes: 0

Views: 2263

Answers (2)

ImportanceOfBeingErnest
ImportanceOfBeingErnest

Reputation: 339052

In plt.plot(x, x, 'k', color=None), you have two color definitions. The first is the positional argument 'k' (black), the second is the keyword argument color=None.

If you leave out the color=None, the positional argument 'k' will be used and the lines will be black. No supprise here.

If you specify color=None, it will overwrite the 'k' argument. Thus color=None will determine the color and that means a color is chosen according to the current color cycle.

To answer the question on how it is possible to distinguish the cases:

First let me note that the way matplotlib manages arguments and keyword arguments to finally decide upon the properties of the lines is rather complex. To see what actually happens, you may start at the plot function and follow the arguments through the various functions and classes. To explain everything here would be too much, so let's look at an example of how it could be done in a simplified way.

The plot method does not look like def plot(x,y,something, color=None), but simply def plot(*args,**kwargs).

Therefore color is not an argument, but a keyword argument. A way the two color arguments could be disinguished would e.g. look like

def plot(*args,**kwargs):
    x = args[0]
    y = args[1]
    if len(args) > 2:
        color = args[2]
    else:
        color = None
    color = kwargs.get("color", color)
    print color

plot([1,2],[3,4], "k", color=None) # prints None
plot([1,2],[3,4], "k")             # prints k
plot([1,2],[3,4], color=None)      # prints None
plot([1,2],[3,4])                  # prints None

Upvotes: 3

stoff
stoff

Reputation: 41

The Parameter 'k' is the color abbreviation for the color black. But when you set color=None it overrides black with None which means matplotlib will use default colors.

matplotlib will then automatically cycle through colors. By default, it cycles through blue, green, red, cyan, magenta, yellow, black.

Edit: In matplotlib 2.0 default color cycle has been changed: source

Upvotes: 3

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