In python-mode.el version 6.0.4, there is this section of code:
;; backward compatibility
(defalias 'py-python-command 'py-shell-name)
(defalias 'py-jpython-command 'py-shell-name)
(defalias 'py-jython-command 'py-shell-name)
(defalias 'py-default-interpreter 'py-shell-name)
(defalias 'python-command 'py-shell-name)
However, "defalias" works only for functions, and all of those symbols are variables, so this "backward-compatibility" section actually has no effect at all, and packages like ipython.el that rely on backward-compatibility become broken. "defvaralias" should be used instead. Although the truly correct function to use is "define-obsolete-variable-alias", which requires a slightly different syntax.