Python List in a For Loop

Question

I am new to Python and had a question about updating a list using a for loop. Here is my code: ``` urls = ['http://www.city-data.com/city/javascript:l("Abbott");' , 'http://www.city-data.com/city/javascript:l("Abernathy");' , 'http://www.city-data.com/city/Abilene-Texas.html' ,'http://www.city-data.com/city/javascript:l("Abram-Perezville");' , 'http://www.city-data.com/city/javascript:l("Ackerly");' , 'http://www.city-data.com/city/javascript:l("Adamsville");', 'http://www.city-data.com/city/Addison-Texas.html'] for url in urls: if "javascript" in url: print url url = url.replace('javascript:l("','').replace('");','-Texas.html') print url for url in urls: if "javascript" in url: url = url.replace('javascript:l("','').replace('");','-Texas.html') print "
" print urls ``` I used the first for loop to check if the syntax was correct, and it worked fine. But the second for loop is the code I want to use, but it's not working properly. How would I go about globally updating the list with…

Accepted Answer

You can update the list items using their index:

```
for i, url in enumerate(urls):
    if "javascript" in url:
        urls[i] = url.replace('javascript:l("','').replace('");','-Texas.html')
```

Another alternative is to use a list comprehension:

```
def my_replace(s):
    return s.replace('javascript:l("','').replace('");','-Texas.html')

urls[:] = [my_replace(url) if "javascript" in url else url for url in urls]
```

Here `urls[:]` means replace all the items of `urls` list with the new list created by the list comprehension.

The reason why your code didn't worked is that you're assigning the variable `url` to something else, and changing one of the reference of an object to point to something else doesn't affect the other references. So, your code is equivalent to:

```
>>> lis = ['aa', 'bb', 'cc']
>>> url = lis[0]                   #create new reference to 'aa'
>>> url = lis[0].replace('a', 'd') #now assign url to a new string that was returned by `lis[0].replace`
>>> url 
'dd'
>>> lis[0]
'aa'
```

Also note that `str.replace` always returns a new copy of string, it never changes the original string because strings are immutable in Python. In case `lis[0]` was a list and you performed any in-place operation on it using `.append`, `.extend` etc, then that would have affected the original list as well.

Python List in a For Loop

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