matplotlib
diff --git a/‎lib/matplotlib/rcsetup.py
Lines changed: 17 additions & 4 deletions b/‎lib/matplotlib/rcsetup.py
Lines changed: 17 additions & 4 deletions
diff --git a/‎lib/matplotlib/tests/test_rcparams.py
Lines changed: 21 additions & 0 deletions b/‎lib/matplotlib/tests/test_rcparams.py
Lines changed: 21 additions & 0 deletions
@@ -461,6 +461,19 @@ def validate_fontsize(s):
 validate_fontsizelist = _listify_validator(validate_fontsize)
 
 
+def validate_fontweight(s):
+    weights = [
+        'ultralight', 'light', 'normal', 'regular', 'book', 'medium', 'roman',
+        'semibold', 'demibold', 'demi', 'bold', 'heavy', 'extra bold', 'black']
+    # Note: Historically, weights have been case-sensitive in Matplotlib
+    if s in weights:
+        return s
+    try:
+        return int(s)
+    except (ValueError, TypeError):
+        raise ValueError(f'{s} is not a valid font weight. %s')
+
+
 def validate_font_properties(s):
     parse_fontconfig_pattern(s)
     return s
@@ -1113,7 +1126,7 @@ def _validate_linestyle(ls):
     'font.style':      ['normal', validate_string],
     'font.variant':    ['normal', validate_string],
     'font.stretch':    ['normal', validate_string],
-    'font.weight':     ['normal', validate_string],
+    'font.weight':     ['normal', validate_fontweight],
     'font.size':       [10, validate_float],      # Base font size in points
     'font.serif':      [['DejaVu Serif', 'Bitstream Vera Serif',
                          'Computer Modern Roman',
@@ -1190,7 +1203,7 @@ def _validate_linestyle(ls):
 
     'axes.titlesize':        ['large', validate_fontsize],  # fontsize of the
                                                             # axes title
-    'axes.titleweight':      ['normal', validate_string],  # font weight of axes title
+    'axes.titleweight':      ['normal', validate_fontweight],  # font weight of axes title
     'axes.titlepad':         [6.0, validate_float],  # pad from axes top to title in points
     'axes.grid':             [False, validate_bool],   # display grid or not
     'axes.grid.which':       ['major', validate_axis_locator],  # set whether the gid are by
@@ -1202,7 +1215,7 @@ def _validate_linestyle(ls):
     'axes.labelsize':        ['medium', validate_fontsize],  # fontsize of the
                                                              # x any y labels
     'axes.labelpad':         [4.0, validate_float],  # space between label and axis
-    'axes.labelweight':      ['normal', validate_string],  # fontsize of the x any y labels
+    'axes.labelweight':      ['normal', validate_fontweight],  # fontsize of the x any y labels
     'axes.labelcolor':       ['black', validate_color],    # color of axis label
     'axes.formatter.limits': [[-7, 7], validate_nseq_int(2)],
                                # use scientific notation if log10
@@ -1340,7 +1353,7 @@ def _validate_linestyle(ls):
     ## figure props
     # figure title
     'figure.titlesize':   ['large', validate_fontsize],
-    'figure.titleweight': ['normal', validate_string],
+    'figure.titleweight': ['normal', validate_fontweight],
 
     # figure size in inches: width by height
     'figure.figsize':    [[6.4, 4.8], validate_nseq_float(2)],
 
@@ -17,6 +17,7 @@
                                 validate_colorlist,
                                 validate_color,
                                 validate_bool,
+                                validate_fontweight,
                                 validate_nseq_int,
                                 validate_nseq_float,
                                 validate_cycler,
@@ -412,6 +413,26 @@ def test_validator_invalid(validator, arg, exception_type):
         validator(arg)
 
 
+@pytest.mark.parametrize('weight, parsed_weight', [
+    ('bold', 'bold'),
+    ('BOLD', ValueError),  # weight is case-sensitive
+    (100, 100),
+    ('100', 100),
+    (np.array(100), 100),
+    # fractional fontweights are not defined. This should actually raise a
+    # ValueError, but historically did not.
+    (20.6, 20),
+    ('20.6', ValueError),
+    ([100], ValueError),
+])
+def test_validate_fontweight(weight, parsed_weight):
+    if parsed_weight is ValueError:
+        with pytest.raises(ValueError):
+            validate_fontweight(weight)
+    else:
+        assert validate_fontweight(weight) == parsed_weight
+
+
 def test_keymaps():
     key_list = [k for k in mpl.rcParams if 'keymap' in k]
     for k in key_list: