In Python, for i in range(5): repeats the indented code five times, assigning i the values 0 through 4. The stop number is excluded. range() supplies a sequence of integers to the loop; it does not create a list of them.
How for i in range(5) works
Consider this loop:
for i in range(5):
print(i)range(5) represents the integers 0, 1, 2, 3, 4. Python’s for statement gets values from that iterable one at a time, assigns each value to i, runs the indented body, and continues until there are no values left. The output is five lines, ending with 4, not 5. The Python tutorial explains that the endpoint is never part of the generated sequence: Python’s range tutorial.
i is an ordinary variable name, not a special Python keyword. It is often used for an index or counter, but choose a more descriptive name such as count or index when that makes the code easier to understand.
Choose the right range() arguments
range() has three forms. The stop value is always excluded; omitted start and step values default to 0 and 1, respectively. See the Python built-in types documentation for the full definition.
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| Form | Values supplied | Example |
|---|---|---|
range(stop) |
Starts at 0 and advances by 1, stopping before stop. |
range(5) supplies 0, 1, 2, 3, 4. |
range(start, stop) |
Starts at start and advances by 1, stopping before stop. |
range(2, 6) supplies 2, 3, 4, 5. |
range(start, stop, step) |
Starts at start and advances by step, stopping before stop. |
range(0, 10, 3) supplies 0, 3, 6, 9. |
Counting down
A negative step counts downward when the start is greater than the stop. For example, range(5, 0, -1) supplies 5, 4, 3, 2, 1; it still excludes the stop value, 0.
Empty ranges and invalid arguments
A range is empty if its first value does not meet the stopping condition. For example, range(5, 0) is empty because the default positive step moves away from the stop. The step cannot be zero; range(1, 5, 0) raises ValueError. Range arguments must be integers or objects that support the integer-index protocol, __index__(), as described in the range documentation.
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A range is not a list
range() returns an immutable sequence object rather than eagerly building a list of every number. Python documents that a range takes the same small amount of memory regardless of how many values it represents: it stores the start, stop, and step, then calculates values as needed. If you specifically need a list—for example, to display the values together or pass them to an API that requires a list—convert it with list(range(5)), which produces [0, 1, 2, 3, 4].
What happens to i inside the loop?
Python assigns the next value from the range to the loop target on each iteration. Changing i in the body does not change the sequence the iterator will provide:
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for i in range(3):
i = 99
print(i)This prints 99 three times. On each iteration, the loop first assigns the next range value to i, then the body replaces it with 99. The iterator itself continues through 0, 1, and 2.
After a nonempty loop finishes, its target remains assigned to the last value supplied by the loop—in this example, 2, even though the body reassigned i during each iteration. If the range is empty, the body never runs and the loop does not assign the target.
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When to use range(), direct iteration, or enumerate()
- Use
range()when the task needs a numeric sequence or specific indices, such as repeating an operation a fixed number of times. - Iterate over a collection directly when you need its values, not their positions. For example, use
for name in names:rather than using indices to retrieve each name. - Use
enumerate()when you need both an item and its index. The Python tutorial’s section onforstatements introduces it for this pattern.
These patterns serve different needs: a range provides numbers, direct iteration provides collection values, and enumerate() pairs collection values with their positions.
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