On Sat, 12 Nov 2005 00:17:59 +0100, jena wrote: > hello, > when i create list of lambdas: > l=[lambda:x.upper() for x in ['a','b','c']] > then l[0]() returns 'C', i think, it should be 'A'
What is wrong with just doing this? L = [x.upper() for x in ['a', 'b', 'c']] py> L = [lambda: x.upper() for x in ['a', 'b', 'c']] py> L [<function <lambda> at 0xf6ff9844>, <function <lambda> at 0xf6ff987c>, <function <lambda> at 0xf6ff98b4>] Why do you want a list of functions? >>> L[0]() 'C' >>> L[1]() 'C' >>> L[2]() 'C' What you have discovered is a bug in your code, caused by some accidental behaviour of Python which will be removed in a new version soon: py> [x.upper() for x in "abc"] ['A', 'B', 'C'] py> x 'c' You can see that the temporary variable x used by the list comprehension is exposed. It shouldn't be, and soon won't be -- it will be an error to refer to the list comp variable outside the list comp. Now watch this: py> x = "it is was a mistake to expose list comprehension variables" py> L[0]() 'IT IS WAS A MISTAKE TO EXPOSE LIST COMPREHENSION VARIABLES' py> L[1]() 'IT IS WAS A MISTAKE TO EXPOSE LIST COMPREHENSION VARIABLES' Do you see what is going on now? Assuming you actually do need a list of *functions*, rather than just the results of those functions, this would be the way to do it: lambda x: x.upper() is an anonymous function which takes input x and returns x converted to upper case. lambda x: x.upper # note the brackets are gone is an anonymous function which takes input x and returns a function (technically, a method) which will return x converted to upper case when called. So the list comprehension you want is: # note all the brackets py> L = [(lambda x: x.upper)(x) for x in ['a', 'b', 'c']] py> L [<built-in method upper of str object at 0xf706a040>, <built-in method upper of str object at 0xf706a0e0>, <built-in method upper of str object at 0xf706ca00>] py> L[0](); L[1](); L[2]() 'A' 'B' 'C' But now that gives us a clue that using lambda just adds too much complication! What we want is the string methods, and we don't need lambda to get them. So we can make it much simpler: py> L = [x.upper for x in ['a', 'b', 'c']] py> L [<built-in method upper of str object at 0xf706a040>, <built-in method upper of str object at 0xf706a0e0>, <built-in method upper of str object at 0xf706ca00>] py> L[0](); L[1](); L[2]() 'A' 'B' 'C' Hope this helps. -- Steven. -- http://mail.python.org/mailman/listinfo/python-list