python - Lambda Replace

Question

What other approach can I use instead of 'lambda'

def calculate_fees(self):
    return sum(map(lambda x: x.fee, self.bookings.values()))

def calculate_donations(self):
    return sum(map(lambda x: x.donation, self.bookings.values()))

def __str__(self):
    output = 'Event: {0} {1} {2}\n'.format(self.event_id, self.event_date, self.venue) + \
             'Miles: {0}\n'.format(self.miles) + \
             'Max Places: {0}\n'.format(self.max_places) + \
             'Basic Fee: £{0}'.format(self.basic_fee)

    if self.bookings:
        output += '\nRunners:\n' + \
                  '\n'.join(map(lambda x: '{0} {1}'.format(x.runner_id, x.club_id), self.bookings.values()))
    return output

Answer 1

The first occurence of lambda could be replaced with a generator:

return sum(x.fee for x in self.bookings.values())

Similar for the second:

return sum(x.donation for x in self.bookings.values())

The third:

'\n'.join('{0} {1}'.format(x.runner_id, x.club_id) for x in self.bookings.values())

Since sum and string.join can deal with a generator, you can use the generator expression directly as an argument to these functions.

Answer 2

Use a generator:

return sum(x.fee for x in self.bookings.values())

Generally, generators and comprehensions improve readability, especially if you have to use a lambda to make map or filter work. Moreover, map and lambda don't play well together performance-wise (Python List Comprehension Vs. Map).