如何删除列表中的项目(如果存在)? [英] How to delete an item in a list if it exists?

问题描述

我正在从带有self.response.get("new_tag")的表单文本字段中获取new_tag，并从带有

I am getting new_tag from a form text field with self.response.get("new_tag") and selected_tags from checkbox fields with

self.response.get_all("selected_tags")

我这样合并它们:

tag_string = new_tag
new_tag_list = f1.striplist(tag_string.split(",") + selected_tags)

(f1.striplist是在列表中的字符串内去除空格的函数.)

(f1.striplist is a function that strips white spaces inside the strings in the list.)

但是如果tag_list为空(没有输入新标签)但有一些selected_tags，则new_tag_list包含一个空字符串" ".

But in the case that tag_list is empty (no new tags are entered) but there are some selected_tags, new_tag_list contains an empty string " ".

例如，来自logging.info:

new_tag
selected_tags[u'Hello', u'Cool', u'Glam']
new_tag_list[u'', u'Hello', u'Cool', u'Glam']

如何摆脱空字符串?

如果列表中有一个空字符串:

If there is an empty string in the list:

>>> s = [u'', u'Hello', u'Cool', u'Glam']
>>> i = s.index("")
>>> del s[i]
>>> s
[u'Hello', u'Cool', u'Glam']

但是如果没有空字符串:

But if there is no empty string:

>>> s = [u'Hello', u'Cool', u'Glam']
>>> if s.index(""):
        i = s.index("")
        del s[i]
    else:
        print "new_tag_list has no empty string"

但这给出了:

Traceback (most recent call last):
  File "<pyshell#30>", line 1, in <module>
    if new_tag_list.index(""):
        ValueError: list.index(x): x not in list

为什么会发生这种情况，我该如何解决?

Why does this happen, and how do I work around it?

推荐答案

1)几乎是英语风格:

使用in运算符测试是否存在，然后应用remove方法.

1) Almost-English style:

Test for presence using the in operator, then apply the remove method.

if thing in some_list: some_list.remove(thing)

remove方法将仅删除第一次出现的thing，要删除所有出现的内容，可以使用while而不是if.

The removemethod will remove only the first occurrence of thing, in order to remove all occurrences you can use while instead of if.

while thing in some_list: some_list.remove(thing)    

• 足够简单，可能是我的选择.对于小的清单(无法抗拒一线)

这种先射击后问问题的态度在Python中很常见.无需预先测试对象是否合适，只需执行操作并捕获相关的异常即可:

This shoot-first-ask-questions-last attitude is common in Python. Instead of testing in advance if the object is suitable, just carry out the operation and catch relevant Exceptions:

try:
    some_list.remove(thing)
except ValueError:
    pass # or scream: thing not in some_list!
except AttributeError:
    call_security("some_list not quacking like a list!")

当然，上面示例中的第二个except子句不仅具有可疑的幽默感，而且完全没有必要(重点是为不熟悉该概念的人说明鸭子式输入法.)

Off course the second except clause in the example above is not only of questionable humor but totally unnecessary (the point was to illustrate duck-typing for people not familiar with the concept).

如果您希望事物发生多次:

If you expect multiple occurrences of thing:

while True:
    try:
        some_list.remove(thing)
    except ValueError:
        break

• 此特定用例有点冗长，但在Python中非常惯用.
• 这比#1表现更好
• PEP 463 针对try/提出了一种较短的语法，但简单用法除外在这里会很方便，但是没有被批准.
• a little verbose for this specific use case, but very idiomatic in Python.
• this performs better than #1
• PEP 463 proposed a shorter syntax for try/except simple usage that would be handy here, but it was not approved.

但是，使用 contextlib的prevent()contextmanager (已引入在python 3.4中)，上述代码可以简化为:

However, with contextlib's suppress() contextmanager (introduced in python 3.4) the above code can be simplified to this:

with suppress(ValueError, AttributeError):
    some_list.remove(thing)

同样，如果您希望事物发生多次:

Again, if you expect multiple occurrences of thing:

with suppress(ValueError):
    while True:
        some_list.remove(thing)

3)功能风格:

在1993年左右，Python得到lambda，reduce()，filter()和map()，由 Lisp 黑客，他们错过了他们并提交了有效的补丁程序*.您可以使用filter从列表中删除元素:

3) Functional style:

Around 1993, Python got lambda, reduce(), filter() and map(), courtesy of a Lisp hacker who missed them and submitted working patches*. You can use filter to remove elements from the list:

is_not_thing = lambda x: x is not thing
cleaned_list = filter(is_not_thing, some_list)

有一个捷径可能对您的情况有用:如果您想过滤出空项目(实际上是bool(item) == False的项目，例如None，零，空字符串或其他空集​​合)，则可以传递无作为第一个参数:

There is a shortcut that may be useful for your case: if you want to filter out empty items (in fact items where bool(item) == False, like None, zero, empty strings or other empty collections), you can pass None as the first argument:

cleaned_list = filter(None, some_list)

• [更新] :在Python 2.x中，filter(function, iterable)以前等同于[item for item in iterable if function(item)](如果第一个参数为None，则为[item for item in iterable if item])；在Python 3.x中，它现在等效于(item for item in iterable if function(item)).细微的区别是过滤器用于返回列表，现在它像生成器表达式一样工作-如果仅遍历已清理的列表并将其丢弃，则可以，但是如果您确实需要列表，则必须将filter()使用list()构造函数调用.
• *这些Lispy风格的构造在Python中被认为有点陌生.在2005年左右， Guido甚至在谈论删除filter -以及同伴map和reduce(它们尚未消失，但reduce已移入 functools 模块，如果您喜欢高级功能，则值得一看).
• [update]: in Python 2.x, filter(function, iterable) used to be equivalent to [item for item in iterable if function(item)] (or [item for item in iterable if item] if the first argument is None); in Python 3.x, it is now equivalent to (item for item in iterable if function(item)). The subtle difference is that filter used to return a list, now it works like a generator expression - this is OK if you are only iterating over the cleaned list and discarding it, but if you really need a list, you have to enclose the filter() call with the list() constructor.
• *These Lispy flavored constructs are considered a little alien in Python. Around 2005, Guido was even talking about dropping filter - along with companions map and reduce (they are not gone yet but reduce was moved into the functools module, which is worth a look if you like high order functions).

列表理解成为Python中列表处理的首选样式，因为 PEP 202 .其背后的理由是，列表推导提供了一种更简洁的方法，可以在当前将使用map()和filter()和/或嵌套循环的情况下创建列表.

List comprehensions became the preferred style for list manipulation in Python since introduced in version 2.0 by PEP 202. The rationale behind it is that List comprehensions provide a more concise way to create lists in situations where map() and filter() and/or nested loops would currently be used.

cleaned_list = [ x for x in some_list if x is not thing ]

生成器表达式是在2.4版中由 PEP 289 引入的.对于实际上不需要(或不想)在内存中创建完整列表的情况(例如，您只想一次遍历一个元素的情况)，生成器表达式是更好的选择.如果仅遍历列表，则可以将生成器表达式视为惰性求值列表理解力:

Generator expressions were introduced in version 2.4 by PEP 289. A generator expression is better for situations where you don't really need (or want) to have a full list created in memory - like when you just want to iterate over the elements one at a time. If you are only iterating over the list, you can think of a generator expression as a lazy evaluated list comprehension:

for item in (x for x in some_list if x is not thing):
    do_your_thing_with(item)

• 请参见此Python历史博客由 GvR 发布.
• 此语法的灵感来自数学中的 set-builder标记.
>
• Python 3还具有 set 和
  • See this Python history blog post by GvR.
  • This syntax is inspired by the set-builder notation in math.
  • Python 3 has also set and dict comprehensions.
  1. 您可能要使用不等式运算符!=而不是is not(区别很重要)
  2. 对于暗示要使用列表副本的方法的批评者:与普遍的看法相反，生成器表达式并不总是比列表理解更有效-请在抱怨之前进行简要介绍
  1. you may want to use the inequality operator != instead of is not (the difference is important)
  2. for critics of methods implying a list copy: contrary to popular belief, generator expressions are not always more efficient than list comprehensions - please profile before complaining

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