# Check if object is True
# bad:

if attr == True:
    print 'It exists!'

# good:
if attr:
    print 'It exists!'


# Try to retrieve object from dictionary
# Bad
if d.has_key('tree'):
     fruit = d['tree']
else:
     fruit = 'default'

# Good
fruit = d.get('tree', 'default')

# Get some items from a list
# Bad
b = []
for item in a:
    if item < 5:
        b.append(item)

# Good
b = [item for item in a if item < 5]