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For-else deserves its own section in the tutorial by nedbat · Pull Request #123946 · python/cpython · GitHub

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For-else deserves its own section in the tutorial #123946

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nedbat merged 3 commits into from
Sep 25, 2024
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For-else deserves its own section in the tutorial #123946

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For-else deserves its own section in the tutorial
nedbat Sep 11, 2024
24be1a9
remove mention of unrolling the loop
nedbat Sep 25, 2024
b42d86c
Update Doc/tutorial/controlflow.rst
nedbat Sep 25, 2024
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85 changes: 54 additions & 31 deletions Doc/tutorial/controlflow.rst
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Expand Up @@ -160,16 +160,52 @@ arguments. In chapter :ref:`tut-structures`, we will discuss in more detail abo

.. _tut-break:

:keyword:`!break` and :keyword:`!continue` Statements, and :keyword:`!else` Clauses on Loops
============================================================================================
:keyword:`!break` and :keyword:`!continue` Statements
=====================================================

The :keyword:`break` statement breaks out of the innermost enclosing
:keyword:`for` or :keyword:`while` loop.
:keyword:`for` or :keyword:`while` loop::

A :keyword:`!for` or :keyword:`!while` loop can include an :keyword:`!else` clause.
>>> for n in range(2, 10):
... for x in range(2, n):
... if n % x == 0:
... print(f"{n} equals {x} * {n//x}")
... break
...
4 equals 2 * 2
6 equals 2 * 3
8 equals 2 * 4
9 equals 3 * 3

The :keyword:`continue` statement continues with the next
iteration of the loop::

>>> for num in range(2, 10):
... if num % 2 == 0:
... print(f"Found an even number {num}")
... continue
... print(f"Found an odd number {num}")
...
Found an even number 2
Found an odd number 3
Found an even number 4
Found an odd number 5
Found an even number 6
Found an odd number 7
Found an even number 8
Found an odd number 9

.. _tut-for-else:

:keyword:`!else` Clauses on Loops
=================================

In a :keyword:`!for` or :keyword:`!while` loop the :keyword:`!break` statement
may be paired with an :keyword:`!else` clause. If the loop finishes without
executing the :keyword:`!break`, the :keyword:`!else` clause executes.

In a :keyword:`for` loop, the :keyword:`!else` clause is executed
after the loop reaches its final iteration.
after the loop finishes its final iteration, that is, if no break occurred.

In a :keyword:`while` loop, it's executed after the loop's condition becomes false.

Expand Down Expand Up @@ -198,32 +234,19 @@ which searches for prime numbers::
9 equals 3 * 3

(Yes, this is the correct code. Look closely: the ``else`` clause belongs to
the :keyword:`for` loop, **not** the :keyword:`if` statement.)

When used with a loop, the ``else`` clause has more in common with the
``else`` clause of a :keyword:`try` statement than it does with that of
:keyword:`if` statements: a :keyword:`try` statement's ``else`` clause runs
when no exception occurs, and a loop's ``else`` clause runs when no ``break``
occurs. For more on the :keyword:`!try` statement and exceptions, see
:ref:`tut-handling`.

The :keyword:`continue` statement, also borrowed from C, continues with the next
iteration of the loop::

>>> for num in range(2, 10):
... if num % 2 == 0:
... print("Found an even number", num)
... continue
... print("Found an odd number", num)
...
Found an even number 2
Found an odd number 3
Found an even number 4
Found an odd number 5
Found an even number 6
Found an odd number 7
Found an even number 8
Found an odd number 9
the ``for`` loop, **not** the ``if`` statement.)

One way to think of the else clause is to imagine it paired with the ``if``
inside the loop. As the loop executes, it will run a sequence like
if/if/if/else. The ``if`` is inside the loop, encountered a number of times. If
the condition is ever true, a ``break`` will happen. If the condition is never
true, the ``else`` clause outside the loop will execute.

When used with a loop, the ``else`` clause has more in common with the ``else``

JelleZijlstra Sep 25, 2024

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Is this paragraph necessary? Many readers of the tutorial will not be familiar with try/else, so I'm not sure this helps many people.

nedbat Sep 25, 2024

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I don't expect people to read purely linearly, and this could help cement the semantics in the reader's mind.
clause of a :keyword:`try` statement than it does with that of ``if``
statements: a ``try`` statement's ``else`` clause runs when no exception
occurs, and a loop's ``else`` clause runs when no ``break`` occurs. For more on
the ``try`` statement and exceptions, see :ref:`tut-handling`.

.. _tut-pass:

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