Equivalence Partitioning

Concept

Split all possible inputs into groups (partitions) where each value in the group behaves the same way. Test one representative value per partition instead of every possible value.

Input space → [invalid low] | [valid] | [invalid high]
                    ↑               ↑           ↑
               one test       one test     one test

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When to Apply

• Numeric ranges (age, price, quantity, score)
• String categories (email format, country code, role name)
• Enumerated inputs (status, type, permission level)
• Any input where the system reacts identically to a whole group of values

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Partitioning Rules

1. Every input value belongs to exactly one partition
2. Partitions must not overlap
3. Together, partitions must cover all possible inputs (valid + invalid)
4. One test per partition is sufficient — more is waste

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Example: Age Validation (18–65)

Partition	Range	Representative	Expected
Invalid low	< 18	10	too_young
Valid	18–65	25	valid
Invalid high	> 65	70	too_old

import pytest


def validate_age(age: int) -> str:
    if age < 18:
        return "too_young"
    if age > 65:
        return "too_old"
    return "valid"


@pytest.mark.parametrize("age, expected", [
    (25, "valid"),      # valid partition: 18–65
    (10, "too_young"),  # invalid partition: < 18
    (70, "too_old"),    # invalid partition: > 65
])
def test_age_equivalence_partitioning(age: int, expected: str) -> None:
    assert validate_age(age) == expected

Three tests instead of hundreds. Each one represents an entire class of inputs.

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Multi-Dimensional Partitioning

When multiple inputs interact, partition each independently, then combine:

def check_edit_permission(role: str, status: str) -> bool:
    return status == "active" and role in ("admin", "editor")


@pytest.mark.parametrize("role, status, can_edit", [
    ("admin", "active", True),
    ("editor", "active", True),
    ("viewer", "active", False),
    ("admin", "inactive", False),
    ("editor", "inactive", False),
    ("viewer", "inactive", False),
])
def test_edit_permission(role: str, status: str, can_edit: bool) -> None:
    assert check_edit_permission(role, status) == can_edit

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Risks

Risk	Description	Mitigation
Wrong partition boundaries	Wrong assumption about where groups split	Review spec carefully; use BVA alongside EP
Missing invalid partitions	Only testing valid inputs	Always define at least one invalid partition
Overlapping partitions	Value belongs to two groups	Redefine partition boundaries clearly

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EP vs BVA

EP picks one representative from inside each partition. BVA focuses on the edges between partitions. Use both together — EP reduces test count, BVA catches off-by-one errors.