Lazily import matplotlib.

A few functions (wfdb.plot_items, wfdb.plot.plot.create_figure,
wfdb.processing.Comparitor.plot) can be used to display graphical
plots using matplotlib.

When the matplotlib module is imported, it does a lot of non-trivial
initialization, which is slow and will sometimes fail in unpredictable
ways (depending on the environment and depending on whether other
nominally-optional dependencies, such as Tk, are installed.)

It's rather absurd for the behavior of importing wfdb to be dependent
on existence or non-existence of a window system, so only import
matplotlib in the few functions that actually use it.

Author: Benjamin Moody

wfdb/plot/plot.py

@@ -1,4 +1,3 @@
-import matplotlib.pyplot as plt
 import numpy as np
 import os
 import pdb
@@ -109,6 +108,8 @@ def plot_items(signal=None, ann_samp=None, ann_sym=None, fs=None,
                         figsize=(10,4), ecg_grids='all')
 
     """
+    import matplotlib.pyplot as plt
+
     # Figure out number of subplots required
     sig_len, n_sig, n_annot, n_subplots = get_plot_dims(signal, ann_samp)
 
@@ -227,6 +228,8 @@ def create_figure(n_subplots, sharex, sharey, figsize):
     axes : list
         The information needed for each subplot.
     """
+    import matplotlib.pyplot as plt
+
     fig, axes = plt.subplots(
         nrows=n_subplots, ncols=1, sharex=sharex, sharey=sharey, figsize=figsize
     )

wfdb/processing/evaluate.py

@@ -1,6 +1,5 @@
 from multiprocessing import cpu_count, Pool
 
-import matplotlib.pyplot as plt
 import numpy as np
 import requests
 
@@ -317,6 +316,8 @@ def plot(self, sig_style='', title=None, figsize=None,
             The axes information for the plot.
 
         """
+        import matplotlib.pyplot as plt
+
         fig = plt.figure(figsize=figsize)
         ax = fig.add_subplot(1, 1, 1)