Ganesh Pal wrote:
>>
>>
>> Whenever you feel the urge to write range(len(whatever)) -- resist that
>> temptation, and you'll end up with better Python code ;)
>>
>>
> Thanks for this suggestion but for my better understanding can explain
> this further even Steve did point the same mistake.
It's not directly a mistake, it's just that programming against a smaller
interface makes your code more flexible and more readable.
Readability:
items = [...]
for i in range(len(items)):
...
Are list entries replaced? We cannot know without inspecting the for suite.
items = [...]
for item in items:
...
Are list entries replaced? No.
Flexibility:
def give_raise(employees, amount):
for i in range(len(items)):
employee = employees[i]
employee.payment += amount
employees = [...]
give_raise(employees, 10)
Now let's adapt the code to raise the payment of some employees:
male_employees = [
employees[i] for i in range(len(employees))
if e.sex == "male"
]
give_raise(male_emplyees, 10)
We need a helper list, basically because there is a len() call inside the
give_raise() function. If we rewrite the function in an idiomatic way
def give_raise(employees, amount):
for employee in employees:
employee.payment += amount
it will become not just more concise, the calling code no longer needs to
build the intermediate list. Instead we save the extra memory and pass a
generator expression:
give_raise((e for e in employees if e.sex == "female"), 10)
In a similar way you can write code that iterates over data that is lazily
read into memory, like lines in a file
# wrong, reads the whole file into memory
lines = open(...).readlines()
for i in range(len(lines)):
process(lines[i])
# idiomatic, read the lines as needed
for line in open(...):
process(line)
or records from a database.
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