Python从列表中删除项目

Question

I'm having some troubles with removing items from a list. I'm looking for a more elegant solution. Preferably a solution in one for-loop or filter.

The objective of the piece of code: remove all empty entries and all entries starting with a '#' from the config handle.

At the moment i'm using:

# Read the config file and put every line in a seperate entry in a list
configHandle = [item.rstrip('\n') for item in open('config.conf')]

# Strip comment items from the configHandle
for item in configHandle:
    if item.startswith('#'):
        configHandle.remove(item)

# remove all empty items in handle
configHandle = filter(lambda a: a != '', configHandle)
print configHandle

This works but I think it is a bit of a nasty solution.

When I try:

# Read the config file and put every line in a seperate entry in a list
configHandle = [item.rstrip('\n') for item in open('config.conf')]

# Strip comment items and empty items from the configHandle
for item in configHandle:
    if item.startswith('#'):
        configHandle.remove(item)
    elif len(item) == 0:
        configHandle.remove(item)

This, however, fails. I cannot figure out why.

Can someone push me in the right direction?

Answer 1

Because You're changing the list while iterating over it. You can use a list comprehension to get ride of this problem:

configHandle = [i for i in configHandle if i and not i.startswith('#')]

Also for opening a file you better to use a with statement that close the file at the end of the block automatically ¹ :

with open('config.conf') as infile :
   configHandle = infile.splitlines()
   configHandle = [line for line in configHandle if line and not line.startswith('#')]

---

_{1. Because there is no guarantee for external links to be collected by garbage-collector. And you need to close them explicitly, which can be done by calling the close() method of a file object, or as mentioned as a more pythonic way use a with statement.}

Answer 2

迭代时不要删除项目，这是一个常见的陷阱

Answer 3

You aren't allowed to modify an item that you're iterating over.

Instead you should use things like filter or list comprehensions.

configHandle = filter(lambda a: (a != '') and not a.startswith('#'), configHandle)

Answer 4

Your filter expression is good; just include the additional condition you're looking for:

configHandle = filter(lambda a: a != '' and not a.startswith('#'), configHandle)

There are other options if you don't want to use filter , but, as has been stated in other answers, it is a very bad idea to attempt to modify a list while you are iterating through it. The answers to this stackoverflow question provides alternatives to using filter to remove from a list based on a condition.