iterating over lines in a file

[nobody]

assume i want to iterate a block of code over every line in a text file,
and that i don't want to snarf the whole thing into memory for fear
of coredumps or whatever. in perl (and many others) there is a simple,
common idiom:

while (line = <FILE>) { block; }

this doesn't seem to have a simple, direct analog in python. searching
around on the web i found a solution at faqts.com (though this newbie
might like to see it explained, but whatever works):

class Reader:
    def __init__(self, source):
        self.source = source
    def readline(self):
        line = self.source.readline()
        self.line = line
        return line             # may be empty, thus false

file = Reader(open("filename")) # i might have got this worng...?

while (file.readline()):
    line = file.line

now, this seems to me like an awful lot of typing just to get around
the fact that assignments in python do not seem to be expressions
returning the value assigned. since that is thus in several other
languages, and since it gives rise to several common idioms of this
type, i can only assume that there must be some good reason for
breaking this pattern in python; i just can't see what it could
possibly be.

could somebody enlighten me, please? and is there any easier way to
iterate over lines in a file without resorting to ugly break statements?