How do commands resolve in bash?

Question

When doing

which python, I am getting /usr/bin/python.

When doing

which python3, I am getting /usr/bin/python3.

However, When doing alias python=/usr/bin/python3, still which python returns /usr/bin/python.

This makes me think there is some more elaborate logic to finding bash commands, but I was not able to look it up, probably I am not aware of its name.

So, how does bash find commands?

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EDIT to answer comments

see output of alias

noam@ML:~/src/uv-car-parts-segmentation$ which python
/usr/bin/python
noam@ML:~/src/uv-car-parts-segmentation$ which python3
/usr/bin/python3
noam@ML:~/src/uv-car-parts-segmentation$ alias python=/usr/bin/python3
noam@ML:~/src/uv-car-parts-segmentation$ alias
alias alert='notify-send --urgency=low -i "$([ $? = 0 ] && echo terminal || echo error)" "$(history|tail -n1|sed -e '\''s/^\s*[0-9]\+\s*//;s/[;&|]\s*alert$//'\'')"'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l='ls -CF'
alias la='ls -A'
alias ll='ls -alF'
alias ls='ls --color=auto'
alias python='/usr/bin/python3'
noam@ML:~/src/uv-car-parts-segmentation$ which python
/usr/bin/python

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Here is the doc about how commands are resolved, which appears not to be the solution to this.

Answer 1

which is usually/often an external command, it doesn't know about shell aliases, or shell functions for that matter. It only looks for the matching command name in PATH.

type python would recognize the alias, and type should work in all POSIXy shells. In at least Bash and some others, type -a can be used to show all matches of the given name. It doesn't track where the alias points to, though, just shows the pathname that would be used if there was no alias:

$ alias python=/usr/bin/python3
$ type -a python
python is aliased to '/usr/bin/python3'
python is /usr/bin/python

See: Why not use "which"? What to use then?

Bash itself would process aliases first, early in the command line processing, then it would check if the first word after all expansions is (1) a function (2) a builtin, or (3) an external command.

(Unless that first word was an unquoted literal keyword like if, in which case it would parsed using the appropriate syntax. You'd need e.g. "if" or /path/to/if to run an external command called if.)