Check if a key exists in a Python list

Question

Suppose I have a list that can have either one or two elements:

mylist=["important", "comment"] 

or

mylist=["important"] 

Then I want to have a variable to work as a flag depending on this 2nd value existing or not.

What's the best way to check if the 2nd element exists?

I already did it using len(mylist). If it is 2, it is fine. It works but I would prefer to know if the 2nd field is exactly "comment" or not.

I then came to this solution:

>>> try: ...      c=a.index("comment") ... except ValueError: ...      print "no such value" ...  >>> if c: ...   print "yeah" ...  yeah 

But looks too long. Do you think it can be improved? I am sure it can but cannot manage to find a proper way from the Python Data Structures Documentation.

Answer 1

You can use the in operator:

'comment' in mylist 

or, if the position is important, use a slice:

mylist[1:] == ['comment'] 

The latter works for lists that are size one, two or longer, and only is True if the list is length 2 and the second element is equal to 'comment':

>>> test = lambda L: L[1:] == ['comment'] >>> test(['important']) False >>> test(['important', 'comment']) True >>> test(['important', 'comment', 'bar']) False 

Answer 2

What about:

len(mylist) == 2 and mylist[1] == "comment" 

For example:

>>> mylist = ["important", "comment"] >>> c = len(mylist) == 2 and mylist[1] == "comment" >>> c True >>> >>> mylist = ["important"] >>> c = len(mylist) == 2 and mylist[1] == "comment" >>> c False