RFC: avoid pyplot interface in lib

nonos/main.py

@@ -14,7 +14,7 @@
 from importlib.metadata import version
 from multiprocessing import Pool
 from pathlib import Path
-from typing import Any, Optional
+from typing import TYPE_CHECKING, Any, Optional
 
 import cblind  # noqa
 import inifix
@@ -42,10 +42,40 @@
 )
 from nonos.styling import set_mpl_style
 
+if TYPE_CHECKING:
+    from matplotlib.backend_bases import FigureCanvasBase
+    from matplotlib.figure import Figure
+
+
 NONOS_VERSION = version("nonos")
 INIFIX_GE_5_0 = Version(version("inifix")) >= Version("5.0.0")
 
 
+def get_non_interactive_figure(fmt: str) -> "Figure":
+    from matplotlib.backends.backend_agg import FigureCanvasAgg
+    from matplotlib.backends.backend_pdf import FigureCanvasPdf
+    from matplotlib.backends.backend_ps import FigureCanvasPS
+    from matplotlib.backends.backend_svg import FigureCanvasSVG
+    from matplotlib.figure import Figure
+
+    FigureCanvas: type[FigureCanvasBase]
+
+    if fmt in ["png", "jpg", "jpeg", "raw", "rgba", "tif", "tiff"]:
+        FigureCanvas = FigureCanvasAgg
+    elif fmt == "pdf":
+        FigureCanvas = FigureCanvasPdf
+    elif fmt in ["ps", "eps"]:
+        FigureCanvas = FigureCanvasPS
+    elif fmt == "svg":
+        FigureCanvas = FigureCanvasSVG
+    else:
+        raise ValueError(f"unknown file format {fmt}")
+
+    fig = Figure()
+    FigureCanvas(fig)
+    return fig
+
+
 # process function for parallelisation purpose with progress bar
 # counterParallel = Value('i', 0) # initialization of a counter
 def process_field(
@@ -75,12 +105,6 @@ def process_field(
     *,
     log_level,
 ):
-    import matplotlib.pyplot as plt
-
-    if not show:
-        # ref https://github.com/matplotlib/matplotlib/issues/28957
-        plt.switch_backend("Agg")
-
     configure_logger(level=log_level)
     set_mpl_style(scaling=scaling)
     if geometry == "unset":
@@ -124,7 +148,13 @@ def process_field(
         # default_plane = ["x","y"]
         plane = default_plane
 
-    fig = plt.figure()
+    if show:
+        import matplotlib.pyplot as plt
+
+        fig = plt.figure()
+    else:
+        fig = get_non_interactive_figure(fmt)
+
     ax = fig.add_subplot(111, polar=False)
     if dim == 1:
         dsop.map(plane[0], rotate_with=planet_file).plot(
@@ -180,14 +210,13 @@ def process_field(
 
     if show:
         plt.show()
+        plt.close(fig)
     else:
         logger.debug("saving plot: started")
         filename = f"{''.join(plane)}_{field}_{'_'.join(operations)}{'_diff' if diff else '_'}{'_log' if log else ''}{on:04d}.{fmt}"
         fig.savefig(filename, bbox_inches="tight", dpi=dpi)
         logger.debug("saving plot: finished ({})", filename)
 
-    plt.close(fig)
-
 
 def get_parser() -> argparse.ArgumentParser:
     parser = argparse.ArgumentParser(

nonos/styling.py

@@ -52,9 +52,9 @@ def scale_mpl(scaling: float) -> None:
 
 def set_mpl_style(scaling: float) -> None:
     if mpl.__version_info__ >= (3, 7):
-        import matplotlib.pyplot as plt
+        import matplotlib.style
 
-        plt.style.use("nonos.default")
+        matplotlib.style.use("nonos.default")
     else:
         # promise mypy this is a Path to get around a broad return type
         # from importlib_resource.files