Use environment variable to identify conda / mamba

Conda and mamba both set an environment variable which refers to the
base environment's executable path, use that in preference to less
reliable methods, but fall back on the other approaches if unable to
locate the executable this way.

Additionally, change the search to look for the bare command name rather
than the command within the top level of the active environment, I'm
dubious this approach works with any current conda / mamba version which
usually place their executable links in a `condabin` directory or
elsewhere not at the same level as the Python executable.

IPython/core/magics/packaging.py

@@ -1,151 +1,164
 """Implementation of packaging-related magic functions.
 """
 #-----------------------------------------------------------------------------
 #  Copyright (c) 2018 The IPython Development Team.
 #
 #  Distributed under the terms of the Modified BSD License.
 #
 #  The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 import functools
+import os
 import re
 import shlex
 import sys
 from pathlib import Path
 
 from IPython.core.magic import Magics, magics_class, line_magic
 
 
 def is_conda_environment(func):
     @functools.wraps(func)
     def wrapper(*args, **kwargs):
         """Return True if the current Python executable is in a conda env"""
         # TODO: does this need to change on windows?
         if not Path(sys.prefix, "conda-meta", "history").exists():
             raise ValueError(
                 "The python kernel does not appear to be a conda environment.  "
                 "Please use ``%pip install`` instead."
             )
         return func(*args, **kwargs)
 
     return wrapper
 
 
 def _get_conda_like_executable(command):
     """Find the path to the given executable
 
     Parameters
     ----------
 
     executable: string
         Value should be: conda, mamba or micromamba
     """
+    # Check for a environment variable bound to the base executable, both conda and mamba
+    # set these when activating an environment.
+    base_executable = "CONDA_EXE"
+    if 'mamba' in command.lower():
+        base_executable = "MAMBA_EXE"
+    if base_executable in os.environ:
+        executable = Path(os.environ[base_executable])
+        if executable.is_file():
+            return str(executable.resolve())
+
     # Check if there is a conda executable in the same directory as the Python executable.
     # This is the case within conda's root environment.
     executable = Path(sys.executable).parent / command
     if executable.is_file():
         return str(executable)
 
     # Otherwise, attempt to extract the executable from conda history.
-    # This applies in any conda environment.
+    # This applies in any conda environment. Parsing this way is error prone because
+    # different versions of conda and mamba include differing cmd values such as
+    # `conda`, `conda-script.py`, or `path/to/conda`, here use the raw command provided.
     history = Path(sys.prefix, "conda-meta", "history").read_text(encoding="utf-8")
     match = re.search(
-        rf"^#\s*cmd:\s*(?P<command>.*{executable})\s[create|install]",
+        rf"^#\s*cmd:\s*(?P<command>.*{command})\s[create|install]",
         history,
         flags=re.MULTILINE,
     )
     if match:
         return match.groupdict()["command"]
 
     # Fallback: assume the executable is available on the system path.
     return command
 
 
 CONDA_COMMANDS_REQUIRING_PREFIX = {
     'install', 'list', 'remove', 'uninstall', 'update', 'upgrade',
 }
 CONDA_COMMANDS_REQUIRING_YES = {
     'install', 'remove', 'uninstall', 'update', 'upgrade',
 }
 CONDA_ENV_FLAGS = {'-p', '--prefix', '-n', '--name'}
 CONDA_YES_FLAGS = {'-y', '--y'}
 
 
 @magics_class
 class PackagingMagics(Magics):
     """Magics related to packaging & installation"""
 
     @line_magic
     def pip(self, line):
         """Run the pip package manager within the current kernel.
 
         Usage:
           %pip install [pkgs]
         """
         python = sys.executable
         if sys.platform == "win32":
             python = '"' + python + '"'
         else:
             python = shlex.quote(python)
 
         self.shell.system(" ".join([python, "-m", "pip", line]))
 
         print("Note: you may need to restart the kernel to use updated packages.")
 
     def _run_command(self, cmd, line):
         args = shlex.split(line)
         command = args[0] if len(args) > 0 else ""
         args = args[1:] if len(args) > 1 else [""]
 
         extra_args = []
 
         # When the subprocess does not allow us to respond "yes" during the installation,
         # we need to insert --yes in the argument list for some commands
         stdin_disabled = getattr(self.shell, 'kernel', None) is not None
         needs_yes = command in CONDA_COMMANDS_REQUIRING_YES
         has_yes = set(args).intersection(CONDA_YES_FLAGS)
         if stdin_disabled and needs_yes and not has_yes:
             extra_args.append("--yes")
 
         # Add --prefix to point conda installation to the current environment
         needs_prefix = command in CONDA_COMMANDS_REQUIRING_PREFIX
         has_prefix = set(args).intersection(CONDA_ENV_FLAGS)
         if needs_prefix and not has_prefix:
             extra_args.extend(["--prefix", sys.prefix])
 
         self.shell.system(" ".join([cmd, command] + extra_args + args))
         print("\nNote: you may need to restart the kernel to use updated packages.")
 
     @line_magic
     @is_conda_environment
     def conda(self, line):
         """Run the conda package manager within the current kernel.
 
         Usage:
           %conda install [pkgs]
         """
         conda = _get_conda_like_executable("conda")
         self._run_command(conda, line)
 
     @line_magic
     @is_conda_environment
     def mamba(self, line):
         """Run the mamba package manager within the current kernel.
 
         Usage:
           %mamba install [pkgs]
         """
         mamba = _get_conda_like_executable("mamba")
         self._run_command(mamba, line)
 
     @line_magic
     @is_conda_environment
     def micromamba(self, line):
         """Run the conda package manager within the current kernel.
 
         Usage:
           %micromamba install [pkgs]
         """
         micromamba = _get_conda_like_executable("micromamba")
         self._run_command(micromamba, line)