Add more details

doc/source/admin/python.md

@@ -3,31 +3,40 @@
 Galaxy's core functionality is currently supported on Python **2.7** (although
 deprecated) and **>=3.5** .
 
-If Galaxy complains about the version of Python you are using, check that
-`python --version` reports a supported version. If this is not the case:
+If Galaxy complains about the version of Python you are using:
 
-1. Completely remove the virtualenv used by Galaxy, e.g. with:
-   `rm -rf /path/to/galaxy/.venv`
+1. Completely remove the Python virtualenv used by Galaxy (which can be
+   configured with the `GALAXY_VIRTUAL_ENV` environment variable and defaults to
+   `.venv`), e.g. with: `rm -rf /path/to/galaxy/.venv`
 
-2. Let Galaxy know the path of the correct version of Python.
+2. If you were using a Python from a conda environment (which can be configured
+   with the `GALAXY_CONDA_ENV` environment variable and defaults to `_galaxy_`),
+   remove it, e.g. with: `conda env remove -n _galaxy_`
 
-    - If you are using Galaxy >= 20.05, just execute:
-      `export GALAXY_PYTHON=/path/to/python`
-    - If instead you are using an older version of Galaxy, you can manipulate
-      your shell's `$PATH` variable to place the correct version first. This can
-      be done for just Python by creating a new directory, adding a symbolic
-      link to python in there, and putting that directory at the front of
-      `$PATH`:
+3. Let Galaxy know which Python to use in one of the following methods:
 
-        ```sh
-        % mkdir ~/galaxy-python
-        % ln -s /path/to/python ~/galaxy-python/python
-        % export PATH=~/galaxy-python:$PATH
-        ```
-        
-    - If using Galaxy with a conda environment, remove conda environment (e.g. ```rm -rf /path/to/conda/__galaxy__```)
+    - If you want to use Python from conda, just activate the `base` environment
+      and Galaxy will create a new conda environment for itself.
+    - Otherwise:
+        1. Make sure a supported version of Python is installed.
+        2. Verify that the Python interpreter you want to use is first in the
+           output of `which -a python3 python` (or `which -a python` if you are
+           using Galaxy <=20.01). If this is not the case:
+           - If you are using Galaxy >= 20.05, just execute:
+             `export GALAXY_PYTHON=/path/to/python`
+           - If instead you are using an older version of Galaxy, you can
+            manipulate your shell's `PATH` variable to place the correct version
+            first. This can be done for just Python by creating a new directory,
+            adding a symbolic link to `python` in there, and putting that
+            directory at the front of `PATH`:
 
-3. Start Galaxy again.
+              ```sh
+              % mkdir ~/galaxy-python
+              % ln -s /path/to/python ~/galaxy-python/python
+              % export PATH=~/galaxy-python:$PATH
+              ```
+
+4. Start Galaxy again.
 
 N.B. If you have compiled your own Python interpreter from source, please ensure
 that the `ssl`, `sqlite3`, `curses` and `bz2` modules were built and can be