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To check whether a number lies between two values in Python, write a chained comparison such as low < number < high. This form excludes both endpoints. To include an endpoint, change its operator to <=. For example, 0 <= score <= 100 accepts both 0 and 100. The rest of this guide covers how to choose each boundary, the cases where the simple check gives a surprising result, and the pandas method for checking many values at once.
Choose the operator for each endpoint
Each endpoint gets its own operator. The choice is independent for the lower and upper bound, which gives four interval shapes:
| Interval | Python expression | Lower bound | Upper bound |
|---|---|---|---|
| Open (exclusive both ends) | low < number < high |
Excluded | Excluded |
| Closed (inclusive both ends) | low <= number <= high |
Included | Included |
| Half-open, lower included | low <= number < high |
Included | Excluded |
| Half-open, upper included | low < number <= high |
Excluded | Included |
The half-open forms are common when ranges must not overlap. For instance, if one tax band runs from 0 up to but not including 10,000 and the next starts at 10,000, the first band uses 0 <= income < 10000, so no income value is counted twice.
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Why a chained comparison is the standard form
Python lets you chain comparison operators. The Python language reference defines x < y <= z as equivalent to x < y and y <= z, with one difference: y is evaluated only once. You can therefore write the interval as it reads in mathematics, and you do not repeat the middle expression. If the middle value is a function call, such as low < get_value() < high, the call runs once.
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You can also write the longer form, low < number and number < high. It gives the same result for simple values and is useful only when the two conditions belong to different parts of your logic. For a single interval, the chained form is clearer.
A working example
score = 72
if 0 <= score <= 100:
print("within the allowed range")
else:
print("out of range")This prints “within the allowed range”. Replace <= with < on either side to exclude that endpoint. A floating-point value works the same way:
temperature = 36.6
if 36.0 < temperature <= 37.5:
print("in the target band")Edge cases that change the result
Reversed bounds return False
The chained check assumes the lower bound is actually lower. If you pass low greater than high, no ordinary number can satisfy both comparisons, so the expression is always False:
low, high = 100, 0
print(0 <= 50 <= 100) # True
print(high <= 50 <= low) # False, but no number satisfies this chainIf your inputs may arrive in either order and you want the range between them, normalize the pair first:
low, high = sorted((low, high))
print(low <= number <= high)Only do this when “between” should mean “between the smaller and larger value”. If a reversed pair signals an input error, validate it and raise an exception instead.
NaN never falls inside an interval
The Python documentation specifies that an ordered comparison involving NaN (not a number) is false. A chained check that includes a NaN therefore returns False, whatever the bounds:
import math
x = float("nan")
print(0 <= x <= 100) # False
print(math.isnan(x)) # TrueIf missing or invalid readings should be handled separately, test for NaN before the interval check.
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Comparisons test the stored binary value of a float. A value that prints as 0.3 may not be exactly equal to the literal 0.3 after arithmetic. If your rule depends on an approximate boundary, define the tolerance in code, for example by comparing against high + 1e-9, and document it. Do not widen the interval silently.
Operand types must be orderable
The chain works only when the values can be ordered against each other. Comparing an integer with a float is fine. Comparing a number with an unrelated type, such as a string, raises a TypeError in Python 3. Convert user input with float() or int() and handle the ValueError that a bad conversion raises before running the check.
Why range() is not a general interval check
The expression number in range(low, high) looks similar, but range() represents a sequence of integers with the stop value excluded. It is useful for loops and for testing integer membership with a half-open interval. It does not express a general numeric interval. A float such as 2.5 is never a member of range(0, 10), and the upper bound is always excluded, so it cannot model a closed interval. For ordinary numeric ranges, including floats or a included upper endpoint, use comparison operators.
Checking many values with pandas
For a pandas Series, a Python loop is slow and unnecessary. The Series.between(left, right, inclusive=...) method returns a Boolean Series with one result per row. Recent pandas versions accept the string values "both", "neither", "left", and "right" for inclusive. Older code that passes True or False uses a deprecated form, so check the version you have installed with pandas.__version__.
import pandas as pd
scores = pd.Series([55, 72, 91, 100])
mask = scores.between(60, 100, inclusive="both")
print(mask.tolist()) # [False, True, True, True]
print(scores[mask])Missing values (NaN) in the Series produce False in the result, consistent with the scalar behavior above. Use the Boolean mask to filter rows or to count matches with mask.sum().
Quick Recap
Choosing the right approach
- Single number, exclusive or inclusive bounds: use
low < number < highorlow <= number <= high. - Half-open bands that must not overlap: choose
<=on one side and<on the other. - Bounds that may arrive reversed: normalize with
sorted()or reject the input. - Integer sequences or loop indexes:
range()is appropriate. - Column of values in a DataFrame or Series: use
between()and keep the Boolean mask.
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