What is the semantics of a Python 2.7 line containing ONLY identifier. I.e. simply
a
or
something
?
If you know the exact place in the Reference, I'd be very pleased. Tnx.
2 Answers
An identifier by itself is a valid expression. An expression by itself on a line is a valid statement.
The full semantic chain is a little more involved. In order to have nice operator precedence, we classify things like "a and b" as technically both an and_test and an or_test. As a result, a simple identifier technically qualifies as over a dozen grammar items
stmt: simple_stmt | compound_stmt
simple_stmt: small_stmt (';' small_stmt)* [';'] NEWLINE
small_stmt: (expr_stmt | del_stmt | pass_stmt | flow_stmt |
import_stmt | global_stmt | nonlocal_stmt | assert_stmt)
expr_stmt: testlist_star_expr (augassign (yield_expr|testlist) |
('=' (yield_expr|testlist_star_expr))*)
testlist_star_expr: (test|star_expr) (',' (test|star_expr))* [',']
test: or_test ['if' or_test 'else' test] | lambdef
or_test: and_test ('or' and_test)*
and_test: not_test ('and' not_test)*
not_test: 'not' not_test | comparison
comparison: expr (comp_op expr)*
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
power: atom trailer* ['**' factor]
atom: ('(' [yield_expr|testlist_comp] ')' |
'[' [testlist_comp] ']' |
'{' [dictorsetmaker] '}' |
NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False')
a stmt can be composed of a single simple_stmt, which can be composed of a simgle small_stmt, which can be composed of a single expr_stmt, and so on, down through testlist_star_expr, test, or_test, and_test, not_test, comparison, expr, xor_expr, and_expr, shift_expr, arith_expr, term, factor, power, atom, and finally NAME.
Comments
It's a simple expression statement: https://docs.python.org/2/reference/simple_stmts.html
1 Comment
Igor Traskunov
Almost there! More precisely, what I was searching for was section 5.2.1. Identifiers (Names) and the line "When the name is bound to an object, evaluation of the atom yields that object. When a name is not bound, an attempt to evaluate it raises a NameError exception."
NameErrorif e.g.aisn't defined).try: ip1_i_im1 except NameError: ip1_i_im1 = "None"for the first time in my life.ip1_1_im1 = "None"before whatever could fail to assign it.