Assert statements

Code Example

Runnable Example in Jac and JacLib

Try it!JacPython

# Assert Statements

with entry {
    # Basic assert
    assert True;
    print("assertion passed");

    # Assert with expression
    x = 10;
    assert x > 5;
    print("x > 5");

    # Assert with message
    assert x < 100, "x must be less than 100";
    print("x < 100");

    # Assert that fails
    try {
        assert False;
    } except AssertionError {
        print("assertion failed");
    }

    # Assert with message that fails
    try {
        assert 1 > 2, "1 is not greater than 2";
    } except AssertionError as e {
        print(f"assertion failed: {e}");
    }

    # Assert with complex expression
    assert len([1, 2, 3]) == 3;
    print("list length is 3");

    # Assert with comparison
    assert "hello" != "world";
    print("strings are different");

    # Assert with membership
    assert 5 in [1, 2, 3, 4, 5];
    print("5 in list");

    # Assert with identity
    assert None is None;
    print("None is None");
}

# Assert Statements

with entry {
    # Basic assert
    assert True;
    print("assertion passed");

    # Assert with expression
    x = 10;
    assert x > 5;
    print("x > 5");

    # Assert with message
    assert x < 100, "x must be less than 100";
    print("x < 100");

    # Assert that fails
    try {
        assert False;
    } except AssertionError {
        print("assertion failed");
    }

    # Assert with message that fails
    try {
        assert 1 > 2, "1 is not greater than 2";
    } except AssertionError as e {
        print(f"assertion failed: {e}");
    }

    # Assert with complex expression
    assert len([1, 2, 3]) == 3;
    print("list length is 3");

    # Assert with comparison
    assert "hello" != "world";
    print("strings are different");

    # Assert with membership
    assert 5 in [1, 2, 3, 4, 5];
    print("5 in list");

    # Assert with identity
    assert None is None;
    print("None is None");
}

from __future__ import annotations
from jaclang.runtimelib.builtin import *
assert True
print('assertion passed')
x = 10
assert x > 5
print('x > 5')
assert x < 100, 'x must be less than 100'
print('x < 100')
try:
    assert False
except AssertionError:
    print('assertion failed')
try:
    assert 1 > 2, '1 is not greater than 2'
except AssertionError as e:
    print(f'assertion failed: {e}')
assert len([1, 2, 3]) == 3
print('list length is 3')
assert 'hello' != 'world'
print('strings are different')
assert 5 in [1, 2, 3, 4, 5]
print('5 in list')
assert None is None
print('None is None')

Jac Grammar Snippet

assert_stmt: KW_ASSERT expression (COMMA expression)?

Description

Assert statements validate conditions at runtime, making your code fail fast when assumptions are violated. They're essential for debugging and testing.

Basic Assert Syntax

The simplest form of assert checks if a condition is true. Line 5 shows the basic usage. If the condition is false, an AssertionError is raised. Line 6 prints a confirmation message after the assertion passes.

Assertions with Expressions

Lines 9-11 demonstrate using expressions in assertions. After setting x = 10, line 10 asserts that x > 5. Since 10 is indeed greater than 5, the assertion passes and line 11 prints confirmation.

Assertions with Custom Messages

Line 14 shows how to add a descriptive error message to your assertion. The message after the comma appears when the assertion fails, making it easier to understand what went wrong. This is particularly helpful when debugging complex conditions.

Handling Assertion Failures

Lines 18-22 demonstrate catching assertion failures with try-except blocks:

Line	Purpose
18-19	Try block contains assertion that will fail
20-21	Except catches AssertionError and handles it

When assert False on line 19 executes, it raises an AssertionError. The except block catches this error and prints "assertion failed" instead of crashing the program.

Capturing Error Messages

Lines 25-29 show how to access the error message from a failed assertion. The as e clause captures the exception object, allowing you to print or log the custom message. This is useful for creating informative error reports.

Assertions with Function Calls

Line 32 demonstrates using function calls within assertions. The assertion evaluates the entire expression. First len([1, 2, 3]) returns 3, then checks if it equals 3. This pattern is common for validating data structures.

Common Assertion Patterns

graph TD
    A[Assert Statement] --> B{Condition Type}
    B -->|Comparison| C[assert x != y]
    B -->|Membership| D[assert 5 in list]
    B -->|Identity| E[assert None is None]
    B -->|Function Result| F[assert len == 3]

The example demonstrates several assertion patterns:

Lines	Pattern	Example	Use Case
36-37	Inequality	assert "hello" != "world"	Verify values differ
40-41	Membership	assert 5 in [1, 2, 3, 4, 5]	Check list/set contents
44-45	Identity	assert None is None	Verify object identity
32-33	Equality	assert len([1, 2, 3]) == 3	Validate computed values

When to Use Assertions

Assertions are perfect for: - Validating function preconditions (input requirements) - Checking function postconditions (output guarantees) - Verifying internal invariants (conditions that should always be true) - Writing test cases

Assertions vs Exceptions

Unlike regular exceptions, assertions should be used for: - Debugging during development - Catching programming errors (not user errors) - Validating assumptions in your code

For user input validation or expected runtime errors, use regular if statements and raise proper exceptions instead.