Ben Finney writes:
Ben Finney <ben+pyt...@benfinney.id.au>
Date:
11/24/2015 04:49 AM
To:
python-list@python.org
George Trojan<george.tro...@noaa.gov> writes:
The following code has bitten me recently:
t=(0,1)
x,y=t if t else 8, 9
print(x, y)
(0, 1) 9
You can simplify this by taking assignment out of the picture::
>>> t = (0, 1)
>>> t if t else 8, 9
((0, 1), 9)
So that's an “expression list” containing a comma. The reference for
expressions tells us::
An expression list containing at least one comma yields a tuple. The
length of the tuple is the number of expressions in the list.
<URL:https://docs.python.org/3/reference/expressions.html#expression-lists>
I was assuming that a comma has the highest order of evaluation
You were? The operator precedence rules don't even mention comma as an
operator, so why would you assume that?
<URL:https://docs.python.org/3/reference/expressions.html#operator-precedence>
What threw me off was the right column in the table stating that
binding or tuple display has the highest precedence. Somewhere else
there is a statement that a comma makes a tuple, not the parentheses, so
the parentheses on the left did not register with me.
that is the expression 8, 9 should make a tuple. Why this is not the
case?
I'm not sure why it's the case that you assumed that
My practical advice: I don't bother trying to remember the complete
operator precedence rules. My simplified precedence rules are:
* ‘+’, ‘-’ have the same precedence.
* ‘*’, ‘/’, ‘//’ have the same precedence.
* For anything else: Use parentheses to explicitly declare the
precedence I want.
Related: When an expression has enough clauses that it's not *completely
obvious* what's going on, break it up by assigning some sub-parts to
temporary well-chosen descriptive names (not ‘t’).
t was just to illustrate my problem, not the actual code. My lesson is
to use parentheses in all cases, maybe with an exception for an obvious
y, x = x, y. In my new C code I always write
if (a) {
f();
}
instead of a valid 2-liner
if (a)
f();
Too many times I added indented g() call, and since I do more Python
than C, the error was not glaringly obvious.
-- \ “It is far better to grasp the universe as it really is than to |
`\ persist in delusion, however satisfying and reassuring.” —Carl |
_o__) Sagan | Ben Finney
George
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