\productioncont{| \token{generator_expression | \token{dict_display}}}

\productioncont{| \token{string_conversion}}

\subsection{Generator expressions\label{genexpr}}

\indexii{generator}{expression}

A generator expression is a compact generator notation in parentheses:

\begin{productionlist}

\production{generator_expression}

{"(" \token{test} \token{genexpr_for} ")"}

\production{genexpr_for}

{"for" \token{expression_list} "in" \token{test}

[\token{genexpr_iter}]}

\production{genexpr_iter}

{\token{genexpr_for} | \token{genexpr_if}}

\production{genexpr_if}

{"if" \token{test} [\token{genexpr_iter}]}

\end{productionlist}

A generator expression yields a new generator object.

\obindex{generator}

\obindex{generator expression}

It consists of a single expression followed by at least one

\keyword{for} clause and zero or more \keyword{for} or \keyword{if}

clauses. The iterating values of the new generator are those that

would be produced by considering each of the \keyword{for} or

\keyword{if} clauses a block, nesting from left to right, and

evaluating the expression to yield a value that is reached the

innermost block for each iteration.

Variables used in the generator expression are evaluated lazily

when the \method{next()} method is called for generator object

(in the same fashion as normal generators). However, the leftmost

\keyword{for} clause is immediately evaluated so that error produced

by it can be seen before any other possible error in the code that

handles the generator expression.

Subsequent \keyword{for} clauses cannot be evaluated immediately since

they may depend on the previous \keyword{for} loop.

For example: \samp{(x*y for x in range(10) for y in bar(x))}.

The parentheses can be omitted on calls with only one argument.

See section \ref{calls} for the detail.