Question: I have an issue that matplotlib appears to not be following the rcparams. This occurs particularly for text: annotations, axis labels, titles, etc. I will note that I am not running Seaborn at the same time (Seaborn is known to interfere with some matplotlib settings). I am using Python 3.7.10, matplotlib 3.3.4, and seaborn 0.11.1 For instance, here is a simple plot statement. plt.plot([1,2,3], [1,2,3]) plt.title("Simple plot")   Notice that the plot title, as well as the numeric tick labels, all turn out bold. If you look at examples online for matplotlib, they all have non-bold as the default. If I display the contents of plt.rcParams, I get the following (see bottom of post). Notice that the relevant items, e.g., 'figure.titleweight': 'normal', 'font.weight': 'normal' are 'normal' rather than 'bold'. Is there some other local setting it is using? I'm trying to get this to work without providing additional settings. The odd thing is that the default font face (DejaVu Sans), seems to not be responsive to changes in the font weight, such that if I enter the command  plt.title("Simple plot", fontdict={'fontweight': 'normal', 'weight': 'normal', 'family': 'sans-serif'})  I get the same result. But if I choose serif font, I get  plt.title("Simple plot", fontdict={'fontweight': 'normal', 'weight': 'normal', 'family': 'serif'})   Now, I want to get the result in sans-serif, but not bold, without additional parameters. I also checked my matplotlibrc file, and there is nothing there that seems to override these defaults. I also tried running plt.rcdefaults(), but it didn't help. Where are these bold settings coming from?  RcParams({'_internal.classic_mode': False, 'agg.path.chunksize': 0, 'animation.avconv_args': [], 'animation.avconv_path': 'avconv', 'animation.bitrate': -1, 'animation.codec': 'h264', 'animation.convert_args': [], 'animation.convert_path': 'convert', 'animation.embed_limit': 20.0, 'animation.ffmpeg_args': [], 'animation.ffmpeg_path': 'ffmpeg', 'animation.frame_format': 'png', 'animation.html': 'none', 'animation.html_args': [], 'animation.writer': 'ffmpeg', 'axes.autolimit_mode': 'data', 'axes.axisbelow': 'line', 'axes.edgecolor': 'black', 'axes.facecolor': 'white', 'axes.formatter.limits': [-5, 6], 'axes.formatter.min_exponent': 0, 'axes.formatter.offset_threshold': 4, 'axes.formatter.use_locale': False, 'axes.formatter.use_mathtext': False, 'axes.formatter.useoffset': True, 'axes.grid': False, 'axes.grid.axis': 'both', 'axes.grid.which': 'major', 'axes.labelcolor': 'black', 'axes.labelpad': 4.0, 'axes.labelsize': 'medium', 'axes.labelweight': 'normal', 'axes.linewidth': 0.8, 'axes.prop_cycle': cycler('color', ['#1f77b4', '#ff7f0e', '#2ca02c', '#d62728', '#9467bd', '#8c564b', '#e377c2', '#7f7f7f', '#bcbd22', '#17becf']), 'axes.spines.bottom': True, 'axes.spines.left': True, 'axes.spines.right': True, 'axes.spines.top': True, 'axes.titlecolor': 'auto', 'axes.titlelocation': 'center', 'axes.titlepad': 6.0, 'axes.titlesize': 'large', 'axes.titleweight': 'normal', 'axes.titley': None, 'axes.unicode_minus': True, 'axes.xmargin': 0.05, 'axes.ymargin': 0.05, 'axes3d.grid': True, 'backend': 'Qt5Agg', 'backend_fallback': True, 'boxplot.bootstrap': None, 'boxplot.boxprops.color': 'black', 'boxplot.boxprops.linestyle': '-', 'boxplot.boxprops.linewidth': 1.0, 'boxplot.capprops.color': 'black', 'boxplot.capprops.linestyle': '-', 'boxplot.capprops.linewidth': 1.0, 'boxplot.flierprops.color': 'black', 'boxplot.flierprops.linestyle': 'none', 'boxplot.flierprops.linewidth': 1.0, 'boxplot.flierprops.marker': 'o', 'boxplot.flierprops.markeredgecolor': 'black', 'boxplot.flierprops.markeredgewidth': 1.0, 'boxplot.flierprops.markerfacecolor': 'none', 'boxplot.flierprops.markersize': 6.0, 'boxplot.meanline': False, 'boxplot.meanprops.color': 'C2', 'boxplot.meanprops.linestyle': '--', 'boxplot.meanprops.linewidth': 1.0, 'boxplot.meanprops.marker': '^', 'boxplot.meanprops.markeredgecolor': 'C2', 'boxplot.meanprops.markerfacecolor': 'C2', 'boxplot.meanprops.markersize': 6.0, 'boxplot.medianprops.color': 'C1', 'boxplot.medianprops.linestyle': '-', 'boxplot.medianprops.linewidth': 1.0, 'boxplot.notch': False, 'boxplot.patchartist': False, 'boxplot.showbox': True, 'boxplot.showcaps': True, 'boxplot.showfliers': True, 'boxplot.showmeans': False, 'boxplot.vertical': True, 'boxplot.whiskerprops.color': 'black', 'boxplot.whiskerprops.linestyle': '-', 'boxplot.whiskerprops.linewidth': 1.0, 'boxplot.whiskers': 1.5, 'contour.corner_mask': True, 'contour.linewidth': None, 'contour.negative_linestyle': 'dashed', 'date.autoformatter.day': '%Y-%m-%d', 'date.autoformatter.hour': '%m-%d %H', 'date.autoformatter.microsecond': '%M:%S.%f', 'date.autoformatter.minute': '%d %H:%M', 'date.autoformatter.month': '%Y-%m', 'date.autoformatter.second': '%H:%M:%S', 'date.autoformatter.year': '%Y', 'date.epoch': '1970-01-01T00:00:00', 'docstring.hardcopy': False, 'errorbar.capsize': 0.0, 'figure.autolayout': False, 'figure.constrained_layout.h_pad': 0.04167, 'figure.constrained_layout.hspace': 0.02, 'figure.constrained_layout.use': False, 'figure.constrained_layout.w_pad': 0.04167, 'figure.constrained_layout.wspace': 0.02, 'figure.dpi': 100.0, 'figure.edgecolor': 'white', 'figure.facecolor': 'white', 'figure.figsize': [6.4, 4.8], 'figure.frameon': True, 'figure.max_open_warning': 20, 'figure.raise_window': True, 'figure.subplot.bottom': 0.11, 'figure.subplot.hspace': 0.2, 'figure.subplot.left': 0.125, 'figure.subplot.right': 0.9, 'figure.subplot.top': 0.88, 'figure.subplot.wspace': 0.2, 'figure.titlesize': 'large', 'figure.titleweight': 'normal', 'font.cursive': ['Apple Chancery', 'Textile', 'Zapf Chancery', 'Sand', 'Script MT', 'Felipa', 'cursive'], 'font.family': ['sans-serif'], 'font.fantasy': ['Comic Neue', 'Comic Sans MS', 'Chicago', 'Charcoal', 'ImpactWestern', 'Humor Sans', 'xkcd', 'fantasy'], 'font.monospace': ['DejaVu Sans Mono', 'Bitstream Vera Sans Mono', 'Computer Modern Typewriter', 'Andale Mono', 'Nimbus Mono L', 'Courier New', 'Courier', 'Fixed', 'Terminal', 'monospace'], 'font.sans-serif': ['DejaVu Sans', 'Bitstream Vera Sans', 'Computer Modern Sans Serif', 'Lucida Grande', 'Verdana', 'Geneva', 'Lucid', 'Arial', 'Helvetica', 'Avant Garde', 'sans-serif'], 'font.serif': ['DejaVu Serif', 'Bitstream Vera Serif', 'Computer Modern Roman', 'New Century Schoolbook', 'Century Schoolbook L', 'Utopia', 'ITC Bookman', 'Bookman', 'Nimbus Roman No9 L', 'Times New Roman', 'Times', 'Palatino', 'Charter', 'serif'], 'font.size': 10.0, 'font.stretch': 'normal', 'font.style': 'normal', 'font.variant': 'normal', 'font.weight': 'normal', 'grid.alpha': 1.0, 'grid.color': '#b0b0b0', 'grid.linestyle': '-', 'grid.linewidth': 0.8, 'hatch.color': 'black', 'hatch.linewidth': 1.0, 'hist.bins': 10, 'image.aspect': 'equal', 'image.cmap': 'viridis', 'image.composite_image': True, 'image.interpolation': 'antialiased', 'image.lut': 256, 'image.origin': 'upper', 'image.resample': True, 'interactive': True, 'keymap.all_axes': ['a'], 'keymap.back': ['left', 'c', 'backspace', 'MouseButton.BACK'], 'keymap.copy': ['ctrl+c', 'cmd+c'], 'keymap.forward': ['right', 'v', 'MouseButton.FORWARD'], 'keymap.fullscreen': ['f', 'ctrl+f'], 'keymap.grid': ['g'], 'keymap.grid_minor': ['G'], 'keymap.help': ['f1'], 'keymap.home': ['h', 'r', 'home'], 'keymap.pan': ['p'], 'keymap.quit': ['ctrl+w', 'cmd+w', 'q'], 'keymap.quit_all': [], 'keymap.save': ['s', 'ctrl+s'], 'keymap.xscale': ['k', 'L'], 'keymap.yscale': ['l'], 'keymap.zoom': ['o'], 'legend.borderaxespad': 0.5, 'legend.borderpad': 0.4, 'legend.columnspacing': 2.0, 'legend.edgecolor': '0.8', 'legend.facecolor': 'inherit', 'legend.fancybox': True, 'legend.fontsize': 'medium', 'legend.framealpha': 0.8, 'legend.frameon': True, 'legend.handleheight': 0.7, 'legend.handlelength': 2.0, 'legend.handletextpad': 0.8, 'legend.labelspacing': 0.5, 'legend.loc': 'best', 'legend.markerscale': 1.0, 'legend.numpoints': 1, 'legend.scatterpoints': 1, 'legend.shadow': False, 'legend.title_fontsize': None, 'lines.antialiased': True, 'lines.color': 'C0', 'lines.dash_capstyle': 'butt', 'lines.dash_joinstyle': 'round', 'lines.dashdot_pattern': [6.4, 1.6, 1.0, 1.6], 'lines.dashed_pattern': [3.7, 1.6], 'lines.dotted_pattern': [1.0, 1.65], 'lines.linestyle': '-', 'lines.linewidth': 1.5, 'lines.marker': 'None', 'lines.markeredgecolor': 'auto', 'lines.markeredgewidth': 1.0, 'lines.markerfacecolor': 'auto', 'lines.markersize': 6.0, 'lines.scale_dashes': True, 'lines.solid_capstyle': 'projecting', 'lines.solid_joinstyle': 'round', 'markers.fillstyle': 'full', 'mathtext.bf': 'sans:bold', 'mathtext.cal': 'cursive', 'mathtext.default': 'it', 'mathtext.fallback': 'cm', 'mathtext.fallback_to_cm': None, 'mathtext.fontset': 'dejavusans', 'mathtext.it': 'sans:italic', 'mathtext.rm': 'sans', 'mathtext.sf': 'sans', 'mathtext.tt': 'monospace', 'mpl_toolkits.legacy_colorbar': True, 'patch.antialiased': True, 'patch.edgecolor': 'black', 'patch.facecolor': 'C0', 'patch.force_edgecolor': False, 'patch.linewidth': 1.0, 'path.effects': [], 'path.simplify': True, 'path.simplify_threshold': 0.111111111111, 'path.sketch': None, 'path.snap': True, 'pcolor.shading': 'flat', 'pdf.compression': 6, 'pdf.fonttype': 3, 'pdf.inheritcolor': False, 'pdf.use14corefonts': False, 'pgf.preamble': '', 'pgf.rcfonts': True, 'pgf.texsystem': 'xelatex', 'polaraxes.grid': True, 'ps.distiller.res': 6000, 'ps.fonttype': 3, 'ps.papersize': 'letter', 'ps.useafm': False, 'ps.usedistiller': None, 'savefig.bbox': None, 'savefig.directory': 'C:/Users/002060756/Downloads', 'savefig.dpi': 'figure', 'savefig.edgecolor': 'auto', 'savefig.facecolor': 'auto', 'savefig.format': 'png', 'savefig.jpeg_quality': 95, 'savefig.orientation': 'portrait', 'savefig.pad_inches': 0.1, 'savefig.transparent': False, 'scatter.edgecolors': 'face', 'scatter.marker': 'o', 'svg.fonttype': 'path', 'svg.hashsalt': None, 'svg.image_inline': True, 'text.antialiased': True, 'text.color': 'black', 'text.hinting': 'force_autohint', 'text.hinting_factor': 8, 'text.kerning_factor': 0, 'text.latex.preamble': '', 'text.latex.preview': False, 'text.usetex': False, 'timezone': 'UTC', 'tk.window_focus': False, 'toolbar': 'toolbar2', 'webagg.address': '127.0.0.1', 'webagg.open_in_browser': True, 'webagg.port': 8988, 'webagg.port_retries': 50, 'xaxis.labellocation': 'center', 'xtick.alignment': 'center', 'xtick.bottom': True, 'xtick.color': 'black', 'xtick.direction': 'out', 'xtick.labelbottom': True, 'xtick.labelsize': 'medium', 'xtick.labeltop': False, 'xtick.major.bottom': True, 'xtick.major.pad': 3.5, 'xtick.major.size': 3.5, 'xtick.major.top': True, 'xtick.major.width': 0.8, 'xtick.minor.bottom': True, 'xtick.minor.pad': 3.4, 'xtick.minor.size': 2.0, 'xtick.minor.top': True, 'xtick.minor.visible': False, 'xtick.minor.width': 0.6, 'xtick.top': False, 'yaxis.labellocation': 'center', 'ytick.alignment': 'center_baseline', 'ytick.color': 'black', 'ytick.direction': 'out', 'ytick.labelleft': True, 'ytick.labelright': False, 'ytick.labelsize': 'medium', 'ytick.left': True, 'ytick.major.left': True, 'ytick.major.pad': 3.5, 'ytick.major.right': True, 'ytick.major.size': 3.5, 'ytick.major.width': 0.8, 'ytick.minor.left': True, 'ytick.minor.pad': 3.4, 'ytick.minor.right': True, 'ytick.minor.size': 2.0, 'ytick.minor.visible': False, 'ytick.minor.width': 0.6, 'ytick.right': False}) 

Answer: This is a known issue documented in font-manager.py.  KNOWN ISSUES  documentation font variant is untested font stretch is incomplete font size is incomplete default font algorithm needs improvement and testing setWeights function needs improvement 'light' is an invalid weight value, remove it.   There seems to be an issue with font-manager.py "scoring" the default 'sans-serif' font-family on your system. The safest way to fix this is to. Back up the cache This is so that you can revert any modifications to the cache if there is an issue. It’s a JSON file on your system with a name here: fm_path = Path( mpl.get_cachedir(), f"fontlist-v{FontManager.__version__}.json")  Modify the cache Option 1 Manually update the JSON file so that the default font weight that is selected for 'sans-serif' is correct. Option 2 Allow the FontManager to rebuild the cache. del matplotlib.font_manager.weight_dict['sans-serif'] matplotlib.font_manager._rebuild()  References https://github.com/matplotlib/matplotlib/issues/5574 https://github.com/matplotlib/matplotlib/blob/main/lib/matplotlib/font_manager.py Matplotlib: Times New Roman appears bold