forward-sexp goes to eol in python mode (but only sometimes)

Question

When I run the command forward-sexp at the beginning of the line in Python mode, my cursor jumps to the end of the line. I assume that the line is a statement, and is therefore considered to be a sexp. Can I modify this behavior so that forward-sexp only jumps to the end of the symbol?

Steps to Reproduce:

Paste the following lines into a buffer in Python mode

def dummy():
    my_count_variable = 0
    (a, b, c) = (1, 2, 3)
    return my_count_variable

Case 1:

• Move the cursor to the m at the beginning of my_count_variable
• Call forward-sexp (C-M-f)
• Notice that the cursor has jumped to the end of the line
• (I would prefer to jump to the end of the symbol here)

Case 2:

• Place the cursor at the beginning of the second line, just under the d in def
• Call forward-sexp (C-M-f)
• Notice that the cursor has moved to the end of my_count_variable
• (This seems inconsistent with Case 1)

Case 3:

• Place the cursor at the first ( on the third line
• Call forward-sexp (C-M-f)
• Notice that the cursor jumps to the end of the first tuple instead of the end of the statement.
• (Again, why not jump to the end of the expression?)

Is there a way to modify the behavior of forward-sexp so that it does not treat a python "statement" as a balanced expression? There seems to be some inconsistency which causes the command to work how I want, but only sometimes (i.e. cases 2 and 3).

Note: I am using the built-in python-mode from python.el

Answer 1

I'd agree it's an inconsistency (w.r.t. other programming modes); I ended up here searching for the same answer.

Ultimately, I found the answer in the python.el source code (version 0.24.2) which contains:

If you prefer cc-mode-like forward-sexp movement, setting forward-sexp-function to nil is enough, You can do that using the python-mode-hook:

(add-hook 'python-mode-hook
        (lambda () (setq forward-sexp-function nil)))

Answer 2

The behavior is correct and the expected one.

forward-sexp

means: Move forward across one balanced expression. However, what should be "balanced"? Obviously this is a very general term. Emacs takes a single word as balanced expression, but not a single operator-sign. Lists are commonly well detected as balanced.

When starting from "(" in your example, call python-nav-end-of-statement.