DOC Add FAQ entry on detecting pyodide at run and build time (#790)

Makefile.envs

@@ -15,6 +15,8 @@ export HOSTPYTHON=$(HOSTPYTHONROOT)/bin/python3
 export TARGETPYTHONROOT=$(PYODIDE_ROOT)/cpython/installs/python-$(PYVERSION)
 export PYTHONINCLUDE=$(PYODIDE_ROOT)/cpython/installs/python-$(PYVERSION)/include/python$(PYMINOR)
 
+# This env variable is used to detect pyodide at build time,
+# do not rename it.
 export PYODIDE_PACKAGE_ABI=1
 
 export SIDE_LDFLAGS=\

docs/faq.md

@@ -18,3 +18,35 @@ In both cases, files need to be served with a web server and cannot be loaded fr
 
 For security reasons JavaScript in the browser is not allowed to load local
 data files. You need to serve them with a web-browser.
+
+## How to detect that code is run with Pyodide?
+
+**At run time**, you can detect that a code is running with Pyodide using,
+```py
+import sys
+
+if "pyodide" in sys.modules:
+   # running in Pyodide
+```
+
+More generally you can detect Python built with Emscripten (which includes
+Pyodide) with,
+```py
+import platform
+
+if platform.system() == 'Emscripten':
+    # running in Pyodide or other Emscripten based build
+```
+
+This however will not work at build time (i.e. in a `setup.py`) due to the way
+the pyodide build system works. It first compiles packages with the host compiler
+(e.g. gcc) and then re-runs the compilation commands with emsdk. So the `setup.py` is
+never run inside the Pyodide environement.
+
+To detect pyodide, **at build time** use,
+```python
+import os
+
+if "PYODIDE_PACKAGE_ABI" in os.environ:
+    # building for Pyodide
+```