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PEP 548 -- More Flexible Loop Control

PEP:548
Title:More Flexible Loop Control
Author:R David Murray
Status:Rejected
Type:Standards Track
Created:05-Sep-2017
Python-Version:3.7
Post-History:05-Aug-2017

Abstract

This PEP proposes enhancing the break and continue statements with an optional boolean expression that controls whether or not they execute. This allows the flow of control in loops to be expressed more clearly and compactly.

Motivation

Quoting from the rejected PEP 315:

It is often necessary for some code to be executed before each evaluation of the while loop condition. This code is often duplicated outside the loop, as setup code that executes once before entering the loop:

<setup code>
while <condition>:
    <loop body>
    <setup code>

That PEP was rejected because no syntax was found that was superior to the following form:

while True:
    <setup code>
    if not <condition>:
        break
    <loop body>

This PEP proposes a superior form, one that also has application to for loops. It is superior because it makes the flow of control in loops more explicit, while preserving Python's indentation aesthetic.

Syntax

The syntax of the break and continue statements are extended as follows:

break_stmt : "break" ["if" expression]
continue_stmt : "continue" ["if" expression]

In addition, the syntax of the while statement is modified as follows:

while_stmt : while1_stmt|while2_stmt
while1_stmt : "while" expression ":" suite
              ["else" ":" suite]
while2_stmt : "while" ":" suite

Semantics

A break if or continue if is executed if and only if expression evaluates to true.

A while statement with no expression loops until a break or return is executed (or an error is raised), as if it were a 'while True` statement. Given that the loop can never terminate except in a way that would not cause an else suite to execute, no else suite is allowed in the expressionless form. If practical, it should also be an error if the body of an expressionless while does not contain at least one break or return statement.

Justification and Examples

The previous "best possible" form:

while True:
    <setup code>
    if not <condition>:
        break
    <loop body>

could be formatted as:

while True:
    <setup code>
    if not <condition>: break
    <loop body>

This is superficially almost identical to the form proposed by this PEP:

while:
    <setup code>
    break if not <condition>
    <loop body>

The significant difference here is that the loop flow control keyword appears first in the line of code. This makes it easier to comprehend the flow of control in the loop at a glance, especially when reading colorized code.

For example, this is a common code pattern, taken in this case from the tarfile module:

while True:
    buf = self._read(self.bufsize)
    if not buf:
        break
    t.append(buf)

Reading this, we either see the break and possibly need to think about where the while is that it applies to, since the break is indented under the if, and then track backward to read the condition that triggers it; or, we read the condition and only afterward discover that this condition changes the flow of the loop.

With the new syntax this becomes:

while:
    buf = self._read(self.bufsize)
    break if not buf
    t.append(buf)

Reading this we first see the``break``, which obviously applies to the while since it is at the same level of indentation as the loop body, and then we read the condition that causes the flow of control to change.

Further, consider a more complex example from sre_parse:

while True:
    c = self.next
    self.__next()
    if c is None:
        if not result:
            raise self.error("missing group name")
        raise self.error("missing %s, unterminated name" % terminator,
                         len(result))
    if c == terminator:
        if not result:
            raise self.error("missing group name", 1)
        break
    result += c
return result

This is the natural way to write this code given current Python loop control syntax. However, given break if, it would be more natural to write this as follows:

while:
    c = self.next
    self.__next()
    break if c is None or c == terminator
    result += c
if not result:
    raise self.error("missing group name")
elif c is None:
    raise self.error("missing %s, unterminated name" % terminator,
                     len(result))
return result

This form moves the error handling out of the loop body, leaving the loop logic much more understandable. While it would certainly be possible to write the code this way using the current syntax, the proposed syntax makes it more natural to write it in the clearer form.

The proposed syntax also provides a natural, Pythonic spelling of the classic repeat ... until <expression> construct found in other languages, and for which no good syntax has previously been found for Python:

while:
    ...
    break if <expression>

The tarfile module, for example, has a couple of "read until" loops like the following:

while True:
    s = self.__read(1)
    if not s or s == NUL:
        break

With the new syntax this would read more clearly:

while:
    s = self.__read(1)
    break if not s or s == NUL

The case for extending this syntax to continue is less strong, but buttressed by the value of consistency.

It is much more common for a continue statement to be at the end of a multiline if suite, such as this example from zipfile

while True:
    try:
        self.fp = io.open(file, filemode)
    except OSError:
        if filemode in modeDict:
            filemode = modeDict[filemode]
            continue
        raise
    break

The only opportunity for improvement the new syntax would offer for this loop would be the omission of the True token.

On the other hand, consider this example from uuid.py:

for i in range(adapters.length):
    ncb.Reset()
    ncb.Command = netbios.NCBRESET
    ncb.Lana_num = ord(adapters.lana[i])
    if win32wnet.Netbios(ncb) != 0:
        continue
    ncb.Reset()
    ncb.Command = netbios.NCBASTAT
    ncb.Lana_num = ord(adapters.lana[i])
    ncb.Callname = '*'.ljust(16)
    ncb.Buffer = status = netbios.ADAPTER_STATUS()
    if win32wnet.Netbios(ncb) != 0:
        continue
    status._unpack()
    bytes = status.adapter_address[:6]
    if len(bytes) != 6:
        continue
    return int.from_bytes(bytes, 'big')

This becomes:

for i in range(adapters.length):
    ncb.Reset()
    ncb.Command = netbios.NCBRESET
    ncb.Lana_num = ord(adapters.lana[i])
    continue if win32wnet.Netbios(ncb) != 0
    ncb.Reset()
    ncb.Command = netbios.NCBASTAT
    ncb.Lana_num = ord(adapters.lana[i])
    ncb.Callname = '*'.ljust(16)
    ncb.Buffer = status = netbios.ADAPTER_STATUS()
    continue if win32wnet.Netbios(ncb) != 0
    status._unpack()
    bytes = status.adapter_address[:6]
    continue if len(bytes) != 6
    return int.from_bytes(bytes, 'big')

This example indicates that there are non-trivial use cases where continue if also improves the readability of the loop code.

It is probably significant to note that all of the examples selected for this PEP were found by grepping the standard library for while True and continue, and the relevant examples were found in the first four modules inspected.

Source: https://github.com/python/peps/blob/master/pep-0548.txt