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Repeat a block of code a fixed number of times
Use a for loop with range(n) when you know how many times the block should run. The loop variable can be _ when you do not need its value:
for _ in range(4):
print("Hello")This prints Hello four times. range(4) yields 0, 1, 2, and 3: the endpoint is excluded, so range(n) yields n values for a positive integer n. A range yields its numbers as needed rather than first building a list. See the Python Tutorial’s explanation of for loops and range.
A for loop is not limited to counting. It can iterate directly over any iterable, including a list of names or a string:
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for name in ["Ada", "Grace", "Guido"]:
print(name)When you are processing items from a collection, iterate over those items directly instead of using an index range unless you specifically need the indexes.
Repeat code while a condition holds
Use while condition: when the number of repetitions depends on a condition, such as reading input until the user enters a stop word. Make sure the loop body can change the condition or use break to exit; otherwise, the loop may never stop.
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attempts += 1Here, incrementing attempts eventually makes the condition false. Use break when a specific event should end a loop early; it works with either for or while. The Python Tutorial’s control-flow guide covers loop exits and iteration behavior.
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Call a function repeatedly
Put the function call inside a loop. Loop over argument values when each call should receive a different argument:
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for name in ["Ada", "Grace", "Guido"]:
greet(name)If you are transforming items rather than performing a side effect, map(function, iterable) can express the repeated calls as an iterator. For a function that takes multiple arguments, itertools.starmap(function, iterable_of_argument_tuples) calls it with each tuple unpacked as arguments. Use an ordinary loop when it makes the behavior easier to read.
Repeat a value or sequence with itertools
The itertools helpers produce iterators; they do not run an arbitrary block of statements. Choose between them based on what should repeat:
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| Need | Use | Behavior |
|---|---|---|
| The same object repeatedly | itertools.repeat(value, count) |
Yields that object the specified number of times. Omitting count makes it unbounded. |
| Every item of an iterable repeatedly | itertools.cycle(iterable) |
Replays the iterable’s elements indefinitely and saves a copy of them. |
| A Cartesian product with repeated dimensions | itertools.product(iterable, repeat=n) |
Repeats the input as product dimensions; it does not replay the input sequence. |
Use repeat for the same value
For example, repeat(2) supplies the number 2 again and again. Pair an unbounded iterator with a finite input when using it in a pipeline:
from itertools import repeat
powers = map(pow, range(5), repeat(2))
print(list(powers)) # [0, 1, 4, 9, 16]range(5) supplies five bases, so map stops after five calls even though repeat(2) has no end. Do not consume an unbounded repeat(value) by itself without a stopping mechanism. The itertools reference documents the optional count.
Use cycle to replay an iterable
cycle(iterable) repeats all elements of its input, preserving their order. It saves a copy of the input elements, so it is a poor fit for a very large or unbounded source. See the Functional Programming HOWTO for its iterator behavior.
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Do not confuse product with replay
product(items, repeat=4) makes a Cartesian product using items as four dimensions; it does not yield the sequence four times. The input iterables are consumed into pools before product iteration, so use finite inputs. Details are in the itertools reference.
Build a repeated list or string
Use sequence multiplication when you want a new repeated sequence rather than repeated execution:
labels = ["draft"] * 3
sound = "ha" * 3 # "hahaha"Strings are immutable, so repeating a string in a list is safe. With a mutable item, multiplication repeats references to the same object:
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rows = [[]] * 3
rows[0].append("x")
print(rows) # [["x"], ["x"], ["x"]]To create independent inner lists, use a comprehension:
rows = [[] for _ in range(3)]Python’s built-in types documentation describes sequence multiplication and the distinct behavior of range, which is not a sequence that supports multiplication.
Choose the method that matches the repetition
- Run statements a known number of times: use
for _ in range(n). - Run statements until a condition changes: use
while, and provide a way for it to stop. - Call a function with different inputs: loop over the inputs; use
maporstarmapwhen an iterator-based transformation suits the code. - Supply one value repeatedly: use
itertools.repeat, bounded by a count or another finite iterator. - Replay the elements of an iterable: use
itertools.cycleonly when its saved copy is practical. - Create a repeated list or string: use sequence multiplication, taking care with mutable references.
If you need to modify a collection while looping over it, avoid casually changing the same collection you are traversing. The Python Tutorial recommends iterating over a copy or building a new collection for such cases.
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