test-structural-invariants

name	test-structural-invariants
description	For data structure validation: test lengths, relationships, constraints that must hold, verify setup is correct.

test-structural-invariants

When to Use

• After building complex data structures
• Verifying precomputed relationships
• Checking initialization is correct
• Invariants that should always hold

When NOT to Use

• Simple data
• No structural constraints
• Runtime performance critical

The Pattern

Assert properties that must be true about your data structures.

# After building structure, verify invariants
def test_structure():
    # Size invariants
    assert len(items) == EXPECTED_SIZE

    # Relationship invariants
    assert all(condition(item) for item in items)

    # Coverage invariants
    assert set(keys) == expected_keys

    # Bidirectional relationship
    for a, bs in graph.items():
        for b in bs:
            assert a in graph[b]  # Symmetric

Example (from pytudes)

# sudoku.py - Sudoku structure invariants
def test():
    """A set of tests that must pass."""
    # Size invariants
    assert len(squares) == 81       # 9x9 grid
    assert len(unitlist) == 27      # 9 rows + 9 cols + 9 boxes

    # Relationship invariants
    assert all(len(units[s]) == 3 for s in squares)   # Each square in 3 units
    assert all(len(peers[s]) == 20 for s in squares)  # Each square has 20 peers

    # Specific structure verification
    assert units['C2'] == [
        ['A2', 'B2', 'C2', 'D2', 'E2', 'F2', 'G2', 'H2', 'I2'],  # Column
        ['C1', 'C2', 'C3', 'C4', 'C5', 'C6', 'C7', 'C8', 'C9'],  # Row
        ['A1', 'A2', , , , , , , ]   
    ]
     peers[] == ([.. peers...])

    ()


 ():
     make_Apowers(, ) == {
        : [],
        : [, , , ],
        : [, , , ],
        ...
    }
     make_Czroots(make_Apowers(, )) == {: , : , : , ...}
      **  +  **   Czroots
      **   Czroots
      **    Czroots


 ():
     (WORDS) == 
     (WORDS.values()) == 
     WORDS.most_common() == [
        (, ), (, ), (, ), ...
    ]

Key Principles

1. Test after construction: Verify build was correct
2. Size invariants: Expected counts
3. Relationship invariants: Things that must be connected
4. Specific examples: Check known cases manually
5. Use all() for universal checks: Clean assertion syntax