* gather argument in plt.scatter?

Question

I am reading this website and found this line of code:

for i in centroids.keys():
    plt.scatter(*centroids[i], color=colmap[i])

I understand * is used to gather an arbitrary length of arguments so we don't have to explicitly specify all the arguments, right?

But why does this person write the * here?

Especially since after removing the * , the line seem to produce the same result.

The complete code:

## Initialisation

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
%matplotlib inline

df = pd.DataFrame({
    'x': [12, 20, 28, 18, 29, 33, 24, 45, 45, 52, 51, 52, 55, 53, 55,         61, 64, 69, 72],
    'y': [39, 36, 30, 52, 54, 46, 55, 59, 63, 70, 66, 63, 58, 23, 14,     8, 19, 7, 24]
})


np.random.seed(200)
k = 3
# centroids[i] = [x, y]
centroids = {
    i+1: [np.random.randint(0, 80), np.random.randint(0, 80)]
    for i in range(k) 
}

fig = plt.figure(figsize=(5, 5))
plt.scatter(df['x'], df['y'], color='k')
colmap = {1: 'r', 2: 'g', 3: 'b'}
for i in centroids.keys():
    plt.scatter(*centroids[i], color=colmap[i])
plt.xlim(0, 80)
plt.ylim(0, 80)
plt.show()

Answer 1

Axes.scatter(x, y, ...) takes two arguments x and y.

When removing the asterix you have one single argument only, hence plt.scatter(centroids[i], color=colmap[i]) should throw a TypeError: scatter() takes at least 2 arguments (1 given) .

The alternative would be to split the argument beforehands,

x,y = centroids[i]
plt.scatter(x,y, color=colmap[i])