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There's a 'short circuit' in shutil.which(), described as 'If we're given a full path which matches the mode and it exists, we're done here.'
It also matches if an executable file of the same name is present in the working directory, although on most Unix-y systems you need ./ to execute such files in a shell (i.e. ./foo, not just foo).
This could fool code calling which() into thinking that a program is installed, when it is not.
If we consider this a bug, one simple fix would be to only allow the short circuit with an absolute path, so the line
 if _access_check(cmd, mode):
would become
 if os.path.isabs(cmd) and _access_check(cmd, mode):