如果迭代器为空，Python迭代器中下一个元素的默认值?

2022-01-10 00:00:00 python iterator

问题描述

我有一个对象列表，我想找到给定方法为某个输入值返回 true 的第一个对象.这在 Python 中相对容易做到:

I have a list of objects, and I would like to find the first one for which a given method returns true for some input value. This is relatively easy to do in Python:

pattern = next(p for p in pattern_list if p.method(input))

但是，在我的应用程序中，通常没有 p.method(input) 为真的这样的 p，因此这将引发 StopIteration 异常.有没有一种不写 try/catch 块的惯用方法来处理这个问题?

However, in my application it is common that there is no such p for which p.method(input) is true, and so this will raise a StopIteration exception. Is there an idiomatic way to handle this without writing a try/catch block?

特别是，用 if pattern is not None 条件来处理这种情况似乎会更干净，所以我想知道是否有办法扩展我对 的定义code>pattern 在迭代器为空时提供 None 值——或者如果有更 Pythonic 的方式来处理整个问题！

In particular, it seems like it would be cleaner to handle that case with something like an if pattern is not None conditional, so I'm wondering if there's a way to expand my definition of pattern to provide a None value when the iterator is empty -- or if there's a more Pythonic way to handle the overall problem!

解决方案

next 接受默认值:

next accepts a default value:

next(...)
    next(iterator[, default])

    Return the next item from the iterator. If default is given and the iterator
    is exhausted, it is returned instead of raising StopIteration.

等等

>>> print next(i for i in range(10) if i**2 == 9)
3
>>> print next(i for i in range(10) if i**2 == 17)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> print next((i for i in range(10) if i**2 == 17), None)
None

请注意，出于语法原因，您必须将 genexp 包含在额外的括号中，否则:

Note that you have to wrap the genexp in the extra parentheses for syntactic reasons, otherwise:

>>> print next(i for i in range(10) if i**2 == 17, None)
  File "<stdin>", line 1
SyntaxError: Generator expression must be parenthesized if not sole argument

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