问题:为什么Pylint认为在条件值中使用len(SEQUENCE)不正确?

考虑以下代码片段:

from os import walk

files = []
for (dirpath, _, filenames) in walk(mydir):
    # more code that modifies files
if len(files) == 0: # <-- C1801
    return None

Pylint使我对有关if语句行的消息感到震惊:

[pylint] C1801:请勿len(SEQUENCE)用作条件值

乍一看,规则C1801在我看来并不十分合理,参考指南中的定义也无法解释为什么这是一个问题。实际上,它彻头彻尾地称其为不正确的用法

len-as-condition(C1801)不要len(SEQUENCE)用作条件值当Pylint检测到内部条件不正确使用len(sequence)时使用。

我的搜索尝试也未能为我提供更深入的解释。我确实知道,序列的length属性可能会被延迟评估,并且__len__可以编程为具有副作用,但是令人怀疑的是,仅此一个问题是否足以使Pylint认为这种用法不正确。因此,在我简单地将项目配置为忽略规则之前,我想知道我的推理中是否缺少某些内容。

什么时候将len(SEQ)用作条件值有问题?Pylint尝试使用C1801避免哪些主要情况?

Considering this code snippet:

from os import walk

files = []
for (dirpath, _, filenames) in walk(mydir):
    # more code that modifies files
if len(files) == 0: # <-- C1801
    return None

I was alarmed by Pylint with this message regarding the line with the if statement:

[pylint] C1801:Do not use len(SEQUENCE) as condition value

The rule C1801, at first glance, did not sound very reasonable to me, and the definition on the reference guide does not explain why this is a problem. In fact, it downright calls it an incorrect use.

len-as-condition (C1801): Do not use len(SEQUENCE) as condition value Used when Pylint detects incorrect use of len(sequence) inside conditions.

My search attempts have also failed to provide me a deeper explanation. I do understand that a sequence’s length property may be lazily evaluated, and that __len__ can be programmed to have side effects, but it is questionable whether that alone is problematic enough for Pylint to call such a use incorrect. Hence, before I simply configure my project to ignore the rule, I would like to know whether I am missing something in my reasoning.

When is the use of len(SEQ) as a condition value problematic? What major situations is Pylint attempting to avoid with C1801?


回答 0

什么时候将len(SEQ)用作条件值有问题?Pylint尝试使用C1801避免哪些主要情况?

使用它并不是真的有问题len(SEQUENCE)-尽管它可能没有效率那么高(请参阅chepner的评论)。无论如何,Pylint会检查代码是否符合PEP 8样式指南,该指南指出

对于序列(字符串,列表,元组),请使用空序列为假的事实。

Yes: if not seq:
     if seq:

No:  if len(seq):
     if not len(seq):

作为偶尔在各种语言之间徘徊的Python程序员,我认为该len(SEQUENCE)结构更具可读性和显式性(“显式优于隐式”)。但是,使用空序列False在布尔上下文中求值的事实被认为更“ Pythonic”。

When is the use of len(SEQ) as a condition value problematic? What major situations is Pylint attempting to avoid with C1801?

It’s not really problematic to use len(SEQUENCE) – though it may not be as efficient (see chepner’s comment). Regardless, Pylint checks code for compliance with the PEP 8 style guide which states that

For sequences, (strings, lists, tuples), use the fact that empty sequences are false.

Yes: if not seq:
     if seq:

No:  if len(seq):
     if not len(seq):

As an occasional Python programmer, who flits between languages, I’d consider the len(SEQUENCE) construct to be more readable and explicit (“Explicit is better then implicit”). However, using the fact that an empty sequence evaluates to False in a Boolean context is considered more “Pythonic”.


回答 1

请注意,使用NumPy数组时,实际上需要使用len(seq)(而不是仅检查seq的bool值)。

a = numpy.array(range(10))
if a:
    print "a is not empty"

导致异常:ValueError:具有多个元素的数组的真值不明确。使用a.any()或a.all()

因此,对于同时使用Python列表和NumPy数组的代码,C1801消息的用处不大。

Note that the use of len(seq) is in fact required (instead of just checking the bool value of seq) when using NumPy arrays.

a = numpy.array(range(10))
if a:
    print "a is not empty"

results in an exception: ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

And hence for code that uses both Python lists and NumPy arrays, the C1801 message is less than helpful.


回答 2

这是pylint中的问题,并且不再视为len(x) == 0不正确。

您不应以裸露 len(x)为条件。比较len(x)反对一个明确的值,如if len(x) == 0if len(x) > 0是PEP 8完全正常和不禁止。

PEP 8

# Correct:
if not seq:
if seq:

# Wrong:
if len(seq):
if not len(seq):

请注意,不禁止明确测试长度Python禅宗指出:

显式胜于隐式。

在这两者之间的选择if not seqif not len(seq),无一不是隐含的,而行为是不同的。但是if len(seq) == 0或者if len(seq) > 0是显式比较,并且在许多情况下是正确的行为。

在pylint中,PR 2815修复了此错误,该错误首先报告为问题2684。它会继续抱怨if len(seq),但不再抱怨if len(seq) > 0。PR已在2019-03-19合并,因此如果您使用的是pylint 2.4(于2019-09-14发布),则不应看到此问题。

This was a issue in pylint, and it no longer considers len(x) == 0 as incorrect.

You should not use a bare len(x) as a condition. Comparing len(x) against an explicit value, such as if len(x) == 0 of if len(x) > 0 is totally fine and not prohibited by PEP 8.

From PEP 8:

# Correct:
if not seq:
if seq:

# Wrong:
if len(seq):
if not len(seq):

Note that explicitly testing for the length is not prohibited. The Zen of Python states:

Explicit is better than implicit.

In the choice between if not seq and if not len(seq), both are implicit but behaviour is different. But if len(seq) == 0 or if len(seq) > 0 are explicit comparisons and in many contexts the correct behaviour.

In pylint, PR 2815 has fixed this bug, first reported as issue 2684. It will continue to complain about if len(seq), but it will no longer complain about if len(seq) > 0. The PR was merged 2019-03-19 so if you are using pylint 2.4 (released 2019-09-14) you should not see this problem.


回答 3

Pylint未能提供我的代码,研究使我转向了这篇文章:

../filename.py:49:11: C1801: Do not use `len(SEQUENCE)` to determine if a sequence is empty (len-as-condition)
../filename.py:49:34: C1801: Do not use `len(SEQUENCE)` to determine if a sequence is empty (len-as-condition)

这是我之前的代码:

def list_empty_folders(directory):
"""The Module Has Been Build to list empty Mac Folders."""
for (fullpath, dirnames, filenames) in os.walk(directory):
    if len(dirnames) == 0 and len(filenames) == 0:
        print("Exists: {} : Absolute Path: {}".format(
            os.path.exists(fullpath), os.path.abspath(fullpath)))

这是我的代码修复之后。通过使用int() attribute,我似乎对Pep8 / Pylint感到满意,并且似乎对我的代码没有负面影响:

def list_empty_folders(directory):
"""The Module Has Been Build to list empty Mac Folders."""
for (fullpath, dirnames, filenames) in os.walk(directory):
    if len(dirnames).__trunc__() == 0 and len(filenames).__trunc__() == 0:
        print("Exists: {} : Absolute Path: {}".format(
            os.path.exists(fullpath), os.path.abspath(fullpath)))

我的修复

通过增加.__trunc__()顺序,似乎已经解决了需求。

我的行为没有区别,但是如果有人知道我所缺少的细节,请告诉我。

Pylint was failing for my code and research led me to this post:

../filename.py:49:11: C1801: Do not use `len(SEQUENCE)` to determine if a sequence is empty (len-as-condition)
../filename.py:49:34: C1801: Do not use `len(SEQUENCE)` to determine if a sequence is empty (len-as-condition)

This was my code before:

def list_empty_folders(directory):
"""The Module Has Been Build to list empty Mac Folders."""
for (fullpath, dirnames, filenames) in os.walk(directory):
    if len(dirnames) == 0 and len(filenames) == 0:
        print("Exists: {} : Absolute Path: {}".format(
            os.path.exists(fullpath), os.path.abspath(fullpath)))

This was after my code fix. By using the int() attribute, I seem to have satisfied the Pep8/Pylint and doesn’t seem to have a negative impact on my code:

def list_empty_folders(directory):
"""The Module Has Been Build to list empty Mac Folders."""
for (fullpath, dirnames, filenames) in os.walk(directory):
    if len(dirnames).__trunc__() == 0 and len(filenames).__trunc__() == 0:
        print("Exists: {} : Absolute Path: {}".format(
            os.path.exists(fullpath), os.path.abspath(fullpath)))

My Fix

By adding .__trunc__() to the sequence it seems to have settled the need.

I do not see a difference in the behaviour, but if anyone knows specifics that I am missing, please let me know.


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