Python range

The Python range type generates a sequence of integers by defining the start and end points of a range. It is typically used with a for loop to iterate over a sequence of numbers.

range() works differently in Python 2 and 3.

In Python 2, there are two functions that allow you to generate sequences of integers range and xrange. These functions are very similar, the main difference being that range returns a list and xrange returns an xrange object.

In Python 3, the xrange function has been removed, and the range function behaves similarly to Python 2 xrange. Python 3 range is not a function, but a type representing an immutable sequence of numbers.

In this article, we’ll cover the basics of the Python 3 range type.

Python range() syntax

The range constructor takes the following form.

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range(stop)
range(start, stop[, step])

The arguments provided to the range constructor must be integers. Floating point numbers and other types are not allowed.

range takes one required argument and two optional arguments. It returns a range object representing the given range, and generates numbers as needed.

Python range(stop)

When only one independent variable is given, range returns a sequence of numbers, in increments of 1, from 0 to stop - 1.

The following range types are available.

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for i in range(5):
    print(i)

The generated sequence of numbers starts with 0 and ends with 4 (5-1).

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5

0
1
2
3
4

If the argument is 0 or a negative integer range, the empty sequence is returned.

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print(list(range(-5)))

We are converting the range object to a list, because range performs inert computation on a sequence of integers. The output is an empty list.

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[]

Python range(start, stop)

When supplied with two arguments, range returns a sequence of numbers from start to stop - 1, in increments of 1.

The following are examples.

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for i in range(3, 5):
    print(i)

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The stop parameter must be greater than start. Otherwise, the sequence is empty.

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print(list(range(5, 3)))

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[]

You can use 0, positive integers and negative integers as parameters.

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print(list(range(-5, -3)))

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[-5, -4]

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print(list(range(-3, 0)))

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[-3, -2, -1]

Python range(start, stop, step)

Given three independent variables, range returns a sequence of numbers from start to stop - 1, incremented or decremented by step.

If step is positive, then range returns the increasing sequence.

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for i in range(0, 26, 5):
    print(i)

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4
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6

0
5
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15
20
25

When incrementing, the stop parameter must be greater than start. Otherwise, the sequence is empty.

If step is negative, then range returns a decreasing sequence.

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for i in range(20, 4, -5):
    print(i)

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4

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15
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5

When decreasing, the stop parameter must be smaller than start. Otherwise, the sequence is empty.

If step is 0, a ValueError exception will be thrown.

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Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: range() arg 3 must not be zero

Conclusion

The Python range type lets you generate sequences of integers. It is mainly used in for loops.