In the program below, we read in the lines of the supplied file as strings, but pass over those which raise a ValueError when converted to float.

# ex4-1-b-swallow-speeds-a.py

f = open('swallow-speeds.txt', 'r')
speeds = []
for line in f.readlines():
    try:
        speeds.append(float(line))
    except (ValueError):
        pass
f.close()

average_speed = sum(speeds)/len(speeds)
print('The average speed of an unladen African swallow is {:.1f} m/s'
            .format(average_speed))

Output:

The average speed of an unladen African swallow is 10.3 m/s