Allow pre-installing pip packages

Author: martinRenou

docs/environment.yml

@@ -6,3 +6,6 @@ dependencies:
   - numpy
   - matplotlib
   - ipycanvas
+  - mpmath
+  - pip:
+    - git+https://github.com/sympy/sympy

jupyterlite_xeus_python/build.py

@@ -11,6 +11,7 @@
 
 from empack.file_packager import pack_environment
 from empack.file_patterns import PkgFileFilter, pkg_file_filter_from_yaml
+from empack.filter_env import write_minimal_conda_meta
 
 import typer
 
@@ -117,6 +118,30 @@ def _create_config(prefix_path):
     os.environ["CONDARC"] = str(prefix_path / ".condarc")
 
 
+def _install_pip_dependencies(prefix_path, dependencies):
+    # Create a fake conda_meta file for each dependency, this prevents empack from filtering out pip dependencies
+    for package in dependencies:
+        # TODO Remove the version specifier from the package name if there is one
+        write_minimal_conda_meta(
+            dict(name=package, version="", build="pip", build_number=0),
+            prefix_path,
+        )
+
+    run(
+        [
+            "pip",
+            "install",
+            *dependencies,
+            "--prefix",
+            prefix_path,
+            "--no-input",
+            "--no-deps",
+            "--verbose",
+        ],
+        check=True,
+    )
+
+
 def build_and_pack_emscripten_env(
     python_version: str = PYTHON_VERSION,
     xeus_python_version: str = "",
@@ -144,6 +169,8 @@ def build_and_pack_emscripten_env(
     if packages or xeus_python_version or environment_file:
         bail_early = False
 
+    pip_dependencies = []
+
     # Process environment.yml file
     if environment_file and Path(environment_file).exists():
         bail_early = False
@@ -166,10 +193,7 @@ def build_and_pack_emscripten_env(
                 if isinstance(dependency, str) and dependency not in specs:
                     specs.append(dependency)
                 elif isinstance(dependency, dict) and dependency.get("pip") is not None:
-                    raise RuntimeError(
-                        """Cannot install pip dependencies in the xeus-python Emscripten environment (yet?).
-                        """
-                    )
+                    pip_dependencies = dependency["pip"]
 
     # Bail early if there is nothing to do
     if bail_early and not force:
@@ -190,6 +214,10 @@ def build_and_pack_emscripten_env(
         # Create emscripten env with the given packages
         create_env(env_name, root_prefix, specs, channels)
 
+        # Install pip dependencies
+        if pip_dependencies:
+            _install_pip_dependencies(prefix_path, pip_dependencies)
+
         pack_kwargs = {}
 
         # Download env filter config