PEP 289 – Generator Expressions | peps.python.org

Abstraction: Python PEP introducing memory-efficient generator expressions in Python 2.4

Key points:

• Generator expressions use syntax (expr for var in iterable) without brackets, avoiding full list allocation; e.g. sum(x*x for x in range(10))
• Only the outermost for-expression is evaluated immediately; remaining expressions are deferred until generator execution
• Must be directly inside parentheses; when used as the sole argument to a function call, no extra parens needed: sum(x**2 for x in range(10))
• Loop variable is not exposed to surrounding scope (unlike list comprehensions in Python 2)
• Performance: early timings showed generators faster, but list comprehensions were optimized in Py2.4; generators win at large data volumes by avoiding cache exhaustion
• Authored by Raymond Hettinger, accepted for Python 2.4; reduces need for itertools.ifilter and itertools.imap

Connections: Python · Raymond Hettinger · Generator Expressions · List Comprehensions

Source: https://www.python.org/dev/peps/pep-0289/