“plt.figure”有什么意义？

Question

import numpy as np
import matplotlib.pyplot as plt

def f(t):
    return np.exp(-t) * np.cos(2*np.pi*t)

t1 = np.arange(0.0, 5.0, 0.1)
t2 = np.arange(0.0, 5.0, 0.02)

plt.figure(1)
plt.subplot(211)
plt.plot(t1, f(t1), 'bo', t2, f(t2), 'k')

plt.subplot(212)
plt.plot(t2, np.cos(2*np.pi*t2), 'r--')
plt.show()

According to offical Matplotlib document( https://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.figure ) A figure function will

"If num is provided, and a figure with this id already exists, make it active, and returns a reference to it. "

I tried do the above on my Ipython without plt.figure, but it showed the two required pictures still.

Answer 1

There are three cases where plt.figure is useful:

1. Obtaining a handle to a figure. In many cases it is useful to have a handle to a figure, ie a variable to store the matplotlib.figure.Figure instance in, such that it can be used later on. Example:

    fig = plt.figure() #... other code fig.autofmt_xdate() 

2. Set figure parameters. An option to set some of the parameters for the figure is to supply them as arguments to plt.figure , eg

    plt.figure(figsize=(10,7), dpi=144) 

3. Create several figures. In order to create several figures in the same script, plt.figure can be used. Example:

    plt.figure() # create a figure plt.plot([1,2,3]) plt.figure() # create another figure plt.plot([4,5,6]) # successive commands are plotted to the new figure 

In many other cases, there would not actually be any need to use plt.figure . Using the pyplot interface, a call to any plotting command is sufficient to create a figure and you can always obtain a handle to the current figure with plt.gcf() .

From another perspective it is often desired not only to have a handle to the figure but also to an axes to plot to. In such cases, the use of plt.subplots is more favorable, fig, ax = plt.subplots() .

Answer 2

The plt.figure gives you a new figure, and depending on the given argument it opens a new figure or not. Compare:

plt.figure(1)
plt.plot(x1, y1)
plt.plot(x2, y2)

to

plt.figure(1)
plt.plot(x1, y1)
plt.figure(2)
plt.plot(x2, y2)

to

plt.figure(1)
plt.plot(x1, y1)
plt.figure(1)
plt.plot(x2, y2)

You will see that the first example is equal to the third, because you retrieve the same figure handle with the second plt.figure(1) call in the third example.