patch matplotlib’s show

src/py.js

@@ -2,17 +2,62 @@ import {pyodide as Pyodide} from "./dependencies.js";
 
 export default async function py(require) {
   const pyodide = await (await require(Pyodide.resolve())).loadPyodide();
+  let patch; // a promise for patching matplotlib (if needed)
   return async function py(strings, ...values) {
-    let globals = {};
+    const globals = {};
     const code = strings.reduce((code, string, i) => {
       if (!(i in values)) return code + string;
       const name = `_${i}`;
       globals[name] = values[i];
       return code + string + name;
     }, "");
-    globals = pyodide.toPy(globals);
-    await pyodide.loadPackagesFromImports(code);
-    const value = await pyodide.runPythonAsync(code, {globals});
+    const imports = findImports(pyodide, code);
+    if (imports.includes("matplotlib") && !patch) await (patch = patchMatplotlib(require, pyodide));
+    if (imports.length) await pyodide.loadPackagesFromImports(code);
+    const value = await pyodide.runPythonAsync(code, {globals: pyodide.toPy(globals)});
     return pyodide.isPyProxy(value) ? value.toJs() : value;
   };
 }
+
+// https://github.com/pyodide/pyodide/blob/1624e4a62445876a2d810fdbfc9ddb69a8321a8e/src/js/api.ts#L119-L125
+function findImports(pyodide, code) {
+  const imports = pyodide.pyodide_py.find_imports(code);
+  try {
+    return imports.toJs();
+  } finally {
+    imports.destroy();
+  }
+}
+
+// Overrides matplotlib’s show function to return a DIV such that when used as
+// the last expression in an Observable cell, the inspector will display it.
+async function patchMatplotlib(require, pyodide) {
+  require.resolve("font-awesome@4.7.0/css/font-awesome.min.css").then(href => {
+    const link = document.createElement("link");
+    link.rel = "stylesheet";
+    link.href = href;
+    document.head.appendChild(link);
+  });
+  await pyodide.loadPackage("matplotlib");
+  await pyodide.runPythonAsync(`from matplotlib import pyplot as plt
+from js import document
+
+_show = plt.show
+
+def create_root_element(self):
+  div = document.createElement("div")
+  document.body.appendChild(div)
+  return div
+
+def show(self):
+  c = plt.gcf().canvas
+  c.create_root_element = create_root_element.__get__(c, c.__class__)
+  _show()
+  div = c.get_element("")
+  if (div.parentNode == document.body):
+    document.body.removeChild(div)
+  return div
+
+plt.show = show.__get__(plt, plt.__class__)
+`);
+}