python列表理解中的语法错误

Question

I have a list of xlib elements like this:

<Choice ID="91149" Total="21"/>
<Choice ID="91139" Total="14"/>
<Choice ID="91159" Total="58"/>

I want to pick the element with ID = 91149. In .NET, I could do something like

element91149 = (from p in choices where p.id=91149).first

I am trying at the syntax in python, working from this example from the python tutorial...

#example from documentation = x for x in 'abracadabra' if x not in 'abc'

My implementation:

h = x for x in results if x.get("ID")=="91149" #invalid syntax

What am I doing wrong?

Answer 1

List comprehensions must be enclosed by square brackets [...] :

h = [x for x in results if x.get("ID")=="91149"]

Just for the record, using normal parenthesis (...) will create a generator expression :

h = (x for x in results if x.get("ID")=="91149")

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However, as @Ashwini mentioned, it is generally very inefficient to read a whole list into memory when all you want is the first item that meets a condition.

Instead, it is usually much faster to use next and a generator expression:

h = next(x for x in results if x.get("ID")=="91149")

Unlike the list comp. (which does it all at once), this solution will yield the items one at a time. Moreover, it will stop once it finds an item that meets the condition.

Be warned though that it will also raise a StopIteration error if it cannot find the item. To avoid this, you can give next a default value to return:

h = next((x for x in results if x.get("ID")=="91149"), None)

In this case, h will be assigned to None if an item that meets the condition cannot be found.