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5.2.5 Generator expressions

A generator expression is a compact generator notation in parentheses:

  generator_expression | ::= | "(" expression genexpr_for ")"
  genexpr_for | ::= | "for" target_list "in" or_test [genexpr_iter]
  genexpr_iter | ::= | genexpr_for | genexpr_if
  genexpr_if | ::= | "if" old_expression [genexpr_iter]

  Download entire grammar as text.

A generator expression yields a new generator object. It consists of a single expression followed by at least one `for` clause and zero or more `for` or `if` clauses. The iterating values of the new generator are those that would be produced by considering each of the `for` or `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 `next()` method is called for generator object (in the same fashion as normal generators). However, the leftmost `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 `for` clauses cannot be evaluated immediately since they may depend on the previous `for` loop. For example: "`(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 5.3.4 for the detail.