Fix some incorrect ValueErrors.

anntzer · anntzer · commit 950a31b75773 · 2019-11-15T11:31:04.000+01:00
... which should have read ValueError(string % args) instead of
ValueError(string, args).

But we may as well use check_getitem and f-strings now.
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -7284,27 +7284,24 @@ def magnitude_spectrum(self, x, Fs=None, Fc=None, window=None,
         if Fc is None:
             Fc = 0
 
-        if scale is None or scale == 'default':
-            scale = 'linear'
-
         spec, freqs = mlab.magnitude_spectrum(x=x, Fs=Fs, window=window,
                                               pad_to=pad_to, sides=sides)
         freqs += Fc
 
-        if scale == 'linear':
+        yunits = cbook._check_getitem(
+            {None: 'energy', 'default': 'energy', 'linear': 'energy',
+             'dB': 'dB'},
+            scale=scale)
+        if yunits == 'energy':
             Z = spec
-            yunits = 'energy'
-        elif scale == 'dB':
+        else:  # yunits == 'dB'
             Z = 20. * np.log10(spec)
-            yunits = 'dB'
-        else:
-            raise ValueError('Unknown scale %s', scale)
 
-        lines = self.plot(freqs, Z, **kwargs)
+        line, = self.plot(freqs, Z, **kwargs)
         self.set_xlabel('Frequency')
         self.set_ylabel('Magnitude (%s)' % yunits)
 
-        return spec, freqs, lines[0]
+        return spec, freqs, line
 
     @_preprocess_data(replace_names=["x"])
     @docstring.dedent_interpd
diff --git a/lib/matplotlib/dviread.py b/lib/matplotlib/dviread.py
@@ -483,7 +483,7 @@ def _post_post(self, _):
 
     @_dispatch(min=250, max=255)
     def _malformed(self, offset):
-        raise ValueError("unknown command: byte %d", 250 + offset)
+        raise ValueError(f"unknown command: byte {250 + offset}")
 
 
 class DviFont:
