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The sum function takes an iterator of numbers think ints or floats and returns a sum of all items in the iterator, in this case the sum of all numbers from 20 to Paste the following into your interpreter session:. The function takes the iterator to excerpt from, the starting index, and the ending index. Remember also that this is returning iterators, not items. If you try to do this, you are basically trying to create an infinite list, and of course, you will run out of memory. This would would mean the Python runtime would be trying to get all items from an infinite iterator, which is not possible.

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The itertools module provides high-performance implementations of many of these, which are well worth learning. The function takes the iterator to excerpt from, the starting index, and the ending index.

Unlike with rangeihertools negative step value is not allowed.

The magic of itertools

Study this session carefully to get a valuable grasp of how these iterator tools interact. The function chr converts this code back to a string character. For example, you can see how the cycle of the hour numbers leads to a cycle of the grouping function results.

If you think of an iterator providing the entirety of another iterator, think yield from.

There irertools more to itertools than I was able to cover in this tutorial, but you now have a basic idea of some of its offerings, and a good look at one of its trickiest but potentially most valuable facilities, groupby. This generator is in a constant state of suspension and resumption until the Python process the interactive interpreter, in this case terminates.

For the most part though, you would probably use generator expressions for such simple manipulations of iterators. With this, you can extract a subset sequence from an iterator, including from an infinite iterator. The idea of map is to take an iterator and generate a derived iterator. In the first line, I use itertools. In this case, it just generates the 0 to 11 sequence over and over again. Here, the outer loop invokes itertools.

You can also join with longer strings as demonstrated in the final couple of lines. View image at full size.

The first irertools is a function that takes two arguments. The sum function takes an iterator of numbers think ints or floats and returns a sum of all items in the iterator, in this case the sum of all numbers from 20 to As with the entire standard library, itertlols can find comprehensive documentation of the modulebut as usual, this is more for a well-versed developer to check on details.

This would would mean the Python runtime would be trying to get all pytthon from an infinite iterator, which is not possible. You can implement the equivalent of this generator expression using the filter function, which again is just good to be aware of.

Remember also that this is returning iterators, not items. Uche Ogbuji Published on June 22, Start by looking at a feature from itertools, which neatly creates pythoh iterators. Of course, you can write your own mapping function.

The first argument to map is the mapping function, the function to apply to each item.

Indeed, I could have done the following. Infinite iterators are actually a useful concept once you start using generators. The idea of reduce is to take an iterator and return a single value derived from all its items. Sign in or register to add and subscribe to comments. When the result of the grouping function changes from one item to the next, a new group is created, with all the items until the next change in grouping function value.

On demand data in Python, Part 2: The magic of itertools

That should help illustrate that you can use generator expressions just like other Python expressions. You get items one at a time, in a forward direction. This is called a reduction, where a series of values is itertoools to one new value. You'll also learn about another powerful but tricky standard library module: