Python tuple assignment and checking in conditional statements [duplicate]

Question

This question already has answers here:

When are parentheses required around a tuple? (3 answers)

Closed 9 years ago.

So I stumbled into a particular behaviour of tuples in python that I was wondering if there is a particular reason for it happening.

While we are perfectly capable of assigning a tuple to a variable without explicitely enclosing it in parentheses:

>>> foo_bar_tuple = "foo","bar"
>>> 

we are not able to print or check in a conditional if statement the variable containing the tuple in the previous fashion (without explicitely typing the parentheses):

>>> print foo_bar_tuple == "foo","bar"
False bar

>>> if foo_bar_tuple == "foo","bar": pass
SyntaxError: invalid syntax
>>> 

>>> print foo_bar_tuple == ("foo","bar")
True
>>> 

>>> if foo_bar_tuple == ("foo","bar"): pass
>>>

Does anyone why? Thanks in advance and although I didn't find any similar topic please inform me if you think it is a possible dublicate. Cheers, Alex

Answer 1

It's because the expressions separated by commas are evaluated before the whole comma-separated tuple (which is an "expression list" in the terminology of the Python grammar). So when you do foo_bar_tuple=="foo", "bar", that is interpreted as (foo_bar_tuple=="foo"), "bar". This behavior is described in the documentation.

You can see this if you just write such an expression by itself:

>>> 1, 2 == 1, 2  # interpreted as "1, (2==1), 2"
(1, False, 2)

The SyntaxError for the unparenthesized tuple is because an unparenthesized tuple is not an "atom" in the Python grammar, which means it's not valid as the sole content of an if condition. (You can verify this for yourself by tracing around the grammar.)

Answer 2

Considering an example of if 1 == 1,2: which should cause SyntaxError, following the full grammar:

if 1 == 1,2:

Using the if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite], we get to shift the if keyword and start parsing 1 == 1,2:

For the test rule, only first production matches:

test: or_test ['if' or_test 'else' test] | lambdef

Then we get:

or_test: and_test ('or' and_test)*

And step down into and_test:

and_test: not_test ('and' not_test)*

Here we just step into not_test at the moment:

not_test: 'not' not_test | comparison

Notice, our input is 1 == 1,2:, thus the first production doesn't match and we check the other one: (1)

comparison: expr (comp_op expr)*

Continuing on stepping down (we take the only the first non-terminal as the zero-or-more star requires a terminal we don't have at all in our input):

expr: xor_expr ('|' xor_expr)*
xor_expr: and_expr ('^' and_expr)*
and_expr: shift_expr ('&' shift_expr)*
shift_expr: arith_expr (('<<'|'>>') arith_expr)*
arith_expr: term (('+'|'-') term)*
term: factor (('*'|'/'|'%'|'//') factor)*
factor: ('+'|'-'|'~') factor | power

Now we use the power production:

power: atom trailer* ['**' factor]
atom: ('(' [yield_expr|testlist_comp] ')' |
   '[' [testlist_comp] ']' |
   '{' [dictorsetmaker] '}' |
   NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False')

And shift NUMBER (1 in our input) and reduce. Now we are back at (1) with input ==1,2: to parse. == matches comp_op:

comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not'

So we shift it and reduce, leaving us with input 1,2: (current parsing output is NUMBER comp_op, we need to match expr now). We repeat the process for the left-hand side, going straight to the atom nonterminal and selecting the NUMBER production. Shift and reduce.

Since , does not match any comp_op we reduce the test non-terminal and receive 'if' NUMBER comp_op NUMBER. We need to match else, elif or : now, but we have , so we fail with SyntaxError.

Answer 3

I think the operator precedence table summarizes this nicely:

You'll see that comparisons come before expressions, which are actually dead last.

in, not in, is, is not,                  Comparisons, including membership tests 
<, <=, >, >=, <>, !=, ==                 and identity tests

...

(expressions...), [expressions...],      Binding or tuple display, list display,
{key: value...}, `expressions...`        dictionary display, string conversion