The good news is: You don’t need to use map or filter at all, you can use generator expressions (lazy) or list comprehensions (eager) instead and thus avoid lambdas completely.
So instead of:
lines = map(lambda x: x.strip(), sys.stdin)
Just use:
# You can use either of those in Python 2 and 3, but map has changed between
# Python 2 and Python 3 so I'll present both equivalents:
lines = (x.strip() for x in sys.stdin) # generator expression (Python 3 map equivalent)
lines = [x.strip() for x in sys.stdin] # list comprehension (Python 2 map equivalent)
It’s probably also faster if you use comprehensions. Very few functions are actually faster when used in map or filter – and using a lambda there is more of an anti-pattern (and slow).
The question only contained an example for map, but you can also replace filter. For example if you want to filter out odd numbers:
filter(lambda x: x%2==0, whatever)
You can use a conditional comprehension instead:
(x for x in whatever if x%2==0)
[x for x in whatever if x%2==0]
You could even combine a map and filter in one comprehension:
(x*2 for x in whatever if x%2==0)
Just consider how that would look like with map and filter:
map(lambda x: x*2, filter(lambda x: x%2==0, whatever))
Note: That doesn’t mean lambda isn’t useful! There are lots of places where lambdas are very handy. Consider the key argument for sorted (and likewise for min and max) or functools.reduce (but better keep away from that function, most of the times a normal for-loop is more readable) or itertools that require a predicate function: itertools.accumulate, itertools.dropwhile, itertools.groupby and itertools.takewhile. Just to name a few examples where a lambda could be useful, there are probably lots of other places as well.