Improve Figure.show for displaying previews in Jupyter notebooks and external viewers

Makefile

@@ -36,7 +36,7 @@ test:
 	@echo ""
 	@cd $(TESTDIR); python -c "import $(PROJECT); $(PROJECT).show_versions()"
 	@echo ""
-	cd $(TESTDIR); pytest $(PYTEST_COV_ARGS) $(PROJECT)
+	cd $(TESTDIR); PYGMT_USE_EXTERNAL_DISPLAY="false" pytest $(PYTEST_COV_ARGS) $(PROJECT)
 	cp $(TESTDIR)/coverage.xml .
 	cp -r $(TESTDIR)/htmlcov .
 	rm -r $(TESTDIR)

doc/Makefile

@@ -31,7 +31,8 @@ html: api
 	@echo
 	@echo "Building HTML files."
 	@echo
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	# Set PYGMT_USE_EXTERNAL_DISPLAY to "false" to disable external display
+	PYGMT_USE_EXTERNAL_DISPLAY="false" $(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 

doc/api/index.rst

@@ -60,6 +60,13 @@ Saving and displaying the figure:
     Figure.show
     Figure.psconvert
 
+Configuring the display settings:
+
+.. autosummary::
+    :toctree: generated
+
+    set_display
+
 
 Data Processing
 ---------------

pygmt/__init__.py

@@ -25,7 +25,7 @@
 # Import modules to make the high-level GMT Python API
 from pygmt import datasets
 from pygmt.accessors import GMTDataArrayAccessor
-from pygmt.figure import Figure
+from pygmt.figure import Figure, set_display
 from pygmt.session_management import begin as _begin
 from pygmt.session_management import end as _end
 from pygmt.src import (

pygmt/figure.py

@@ -6,9 +6,10 @@
 from tempfile import TemporaryDirectory
 
 try:
-    from IPython.display import Image
-except ImportError:
-    Image = None
+    import IPython
+except KeyError:
+    IPython = None  # pylint: disable=invalid-name
+
 
 from pygmt.clib import Session
 from pygmt.exceptions import GMTError, GMTInvalidInput
@@ -25,6 +26,23 @@
 # This is needed for the sphinx-gallery scraper in pygmt/sphinx_gallery.py
 SHOWED_FIGURES = []
 
+# Configurations for figure display
+SHOW_CONFIG = {
+    "method": "external",  # Open in an external viewer by default
+}
+
+# Show figures in Jupyter notebooks if available
+if IPython:
+    get_ipython = IPython.get_ipython()  # pylint: disable=invalid-name
+    if get_ipython and "IPKernelApp" in get_ipython.config:  # Jupyter Notebook enabled
+        SHOW_CONFIG["method"] = "notebook"
+
+# Set environment variable PYGMT_USE_EXTERNAL_DISPLAY to 'false' to disable
+# external display. Use it when running the tests and building the docs to
+# avoid popping up windows.
+if os.environ.get("PYGMT_USE_EXTERNAL_DISPLAY", "true").lower() == "false":
+    SHOW_CONFIG["method"] = "none"
+
 
 class Figure:
     """
@@ -235,62 +253,72 @@ def savefig(
         if show:
             launch_external_viewer(fname)
 
-    def show(self, dpi=300, width=500, method="static"):
+    def show(self, dpi=300, width=500, method=None):
         """
         Display a preview of the figure.
 
-        Inserts the preview in the Jupyter notebook output. You will need to
-        have IPython installed for this to work. You should have it if you are
-        using the notebook.
+        Inserts the preview in the Jupyter notebook output if available,
+        otherwise opens it in the default viewer for your operating system
+        (falls back to the default web browser).
+
+        :func:`pygmt.set_display` can select the default display method
+        (**notebook**, **external**, or **none**).
+
+        The ``method`` parameter can also override the default display method
+        for the current figure. Parameters ``dpi`` and ``width`` can be used
+        to control the resolution and dimension of the figure in the notebook.
+
+        Note: The external viewer can be disabled by setting the
+        PYGMT_USE_EXTERNAL_DISPLAY environment variable to **false**.
+        This is useful when running unit tests and building the documentation
+        in consoles without a Graphical User Interface.
 
-        If ``method='external'``, makes PDF preview instead and opens it in the
-        default viewer for your operating system (falls back to the default web
-        browser). Note that the external viewer does not block the current
-        process, so this won't work in a script.
+        Note that the external viewer does not block the current process.
 
         Parameters
         ----------
         dpi : int
-            The image resolution (dots per inch).
+            The image resolution (dots per inch) in Jupyter notebooks.
         width : int
-            Width of the figure shown in the notebook in pixels. Ignored if
-            ``method='external'``.
+            The image width (in pixels) in Jupyter notebooks.
         method : str
-            How the figure will be displayed. Options are (1) ``'static'``: PNG
-            preview (default); (2) ``'external'``: PDF preview in an external
-            program.
+            How the current figure will be displayed. Options are
 
-        Returns
-        -------
-        img : IPython.display.Image
-            Only if ``method != 'external'``.
+            - **external**: PDF preview in an external program [default]
+            - **notebook**: PNG preview [default in Jupyter notebooks]
+            - **none**: Disable image preview
         """
         # Module level variable to know which figures had their show method
         # called. Needed for the sphinx-gallery scraper.
         SHOWED_FIGURES.append(self)
 
-        if method not in ["static", "external"]:
-            raise GMTInvalidInput("Invalid show method '{}'.".format(method))
-        if method == "external":
-            pdf = self._preview(fmt="pdf", dpi=dpi, anti_alias=False, as_bytes=False)
-            launch_external_viewer(pdf)
-            img = None
-        elif method == "static":
-            png = self._preview(
-                fmt="png", dpi=dpi, anti_alias=True, as_bytes=True, transparent=True
+        # Set the display method
+        if method is None:
+            method = SHOW_CONFIG["method"]
+
+        if method not in ["external", "notebook", "none"]:
+            raise GMTInvalidInput(
+                (
+                    f"Invalid display method '{method}', "
+                    "should be either 'notebook', 'external', or 'none'."
+                )
             )
-            if Image is None:
+
+        if method in ["notebook", "none"]:
+            if IPython is None:
                 raise GMTError(
-                    " ".join(
-                        [
-                            "Cannot find IPython.",
-                            "Make sure you have it installed",
-                            "or use 'method=\"external\"' to open in an external viewer.",
-                        ]
+                    (
+                        "Notebook display is selected, but IPython is not available. "
+                        "Make sure you have IPython installed, "
+                        "or run the script in a Jupyter notebook."
                     )
                 )
-            img = Image(data=png, width=width)
-        return img
+            png = self._preview(fmt="png", dpi=dpi, anti_alias=True, as_bytes=True)
+            IPython.display.display(IPython.display.Image(data=png, width=width))
+
+        if method == "external":
+            pdf = self._preview(fmt="pdf", dpi=dpi, anti_alias=False, as_bytes=False)
+            launch_external_viewer(pdf)
 
     def shift_origin(self, xshift=None, yshift=None):
         """
@@ -396,3 +424,27 @@ def _repr_html_(self):
         subplot,
         text,
     )
+
+
+def set_display(method=None):
+    """
+    Set the display method.
+
+    Parameters
+    ----------
+    method : str or None
+        The method to display an image. Choose from:
+
+        - **external**: PDF preview in an external program [default]
+        - **notebook**: PNG preview [default in Jupyter notebooks]
+        - **none**: Disable image preview
+    """
+    if method in ["notebook", "external", "none"]:
+        SHOW_CONFIG["method"] = method
+    elif method is not None:
+        raise GMTInvalidInput(
+            (
+                f"Invalid display mode '{method}', "
+                "should be either 'notebook', 'external' or 'none'."
+            )
+        )

pygmt/tests/test_figure.py

@@ -8,7 +8,7 @@
 import numpy as np
 import numpy.testing as npt
 import pytest
-from pygmt import Figure
+from pygmt import Figure, set_display
 from pygmt.exceptions import GMTInvalidInput
 
 
@@ -142,8 +142,7 @@ def test_figure_show():
     """
     fig = Figure()
     fig.basemap(region="10/70/-300/800", projection="X3i/5i", frame="af")
-    img = fig.show(width=800)
-    assert img.width == 800
+    fig.show()
 
 
 @pytest.mark.mpl_image_compare
@@ -175,3 +174,12 @@ def test_figure_show_invalid_method():
     fig.basemap(region="10/70/-300/800", projection="X3i/5i", frame="af")
     with pytest.raises(GMTInvalidInput):
         fig.show(method="test")
+
+
+def test_figure_set_display_invalid():
+    """
+    Test to check if an error is raised when an invalid method is passed to
+    set_display.
+    """
+    with pytest.raises(GMTInvalidInput):
+        set_display(method="invalid")