CSC/ECE 517 Fall 2007/wiki3 1 sa

Support for assertions in various o-o programming languages

Topic :Compare the support for assertions in various o-o programming languages. How well is it integrated with the language (instead of being supplied by libraries)? How many kinds of assertions are supported? How are assertions used in the various flavors of XUnit testing frameworks?

Introduction

Definition

"In computer programming, an assertion is a predicate (i.e., a true–false statement) placed in a program to indicate that the developer thinks that the predicate is always true at that place. Assertions are used to help specify programs and to reason about program correctness. For example, a precondition — an assertion placed at the beginning of a section of code — determines the set of states under which the code is expected to be executed. A postcondition — placed at the end — describes the expected state at the end of execution. "[1]

Assertions are essentially a way to implement testing in computer programming and are specified by the programmer to check for correctness in a program. Assertions are a systematic way to check that the internal state of a program is as the programmer expected, with the goal of catching bugs. In particular, they are good for catching false assumptions that were made while writing the code.

Example

Following is a very simple example of an assertion in Java.

if (i % 4 == 0) {
       ...
   } else if (i % 4 == 1) {
       ...
   } else if (i % 4 == 2) {
       ...
   } else {
       assert i % 4 == 3 : i;
       ...
   }

In the above example we know that if the first three conditions are false then i % 4 has to evaluate to 3 and this is asserted using assert. Note that even in this case the assertion may fail if i is negative.

Usage

• Design by Contract: Assertions may be specified at the class level (invariants) or on the method level (preconditions and postconditions). An invariant is a correctness condition imposed on a class, i.e., a condition that must not be violated by any method of a class. A precondition is associated with a particular method and imposes a correctness condition on the client of the method: i.e., the client must ensure that the precondition is fulfilled; otherwise the method is not executed. A postcondition is also associated with a particular method, but it imposes a correctness condition on the implementation of the method; a violation of a postcondition indicates an error in the implementation of the method.
• Runtime checking: Assertions that are a part of the implementation and can be checked at run time for correctness. This is a very common usage of assertions.
• Development Cycle: During the development cycle, assertions are used as a powerful debugging tool. The point at which the assertion failure occurs teh programmer is immediately notified so that he can go ahead and fix the bug.
• Static assertions: Assertions that are checked at compile time are called static assertions.

Support for assertions in various o-o programming languages

intro

Depending on the programming language, assertions can either be a part of the design process or they could be statements which are checked at runtime. In this page we are trying to research the support for assertions present in varios o-o programming languages. The research is based on two main criteria:

1. How well the support for assertions is integrated with the language

There are several ways in which assertions are supported iin different languages. Listed below are few strategies for providing this support.

• Built-in support: This means that support is directly included in the programming language. The programming language contains language constructs to formulate assertions in one way or another. The syntactical correctness of assertions is checked directly by the compiler. In addition a runtime environment must be available to perform the runtime assertion checks and also should be able to selectively enable and disable assertion checking.
• Preprocessing: The general idea here is to formulate assertions separate from the program or to include the assertions as comments. A preprocessor is used to weave the assertions into the program or to transform the comments containing assertion formulas into programming language code. The main advantage of this approach is the separation of programming logic and contracts. This is important in cases, where the programming language itself does not support assertions and the programming language must not be altered. The main disadvantage of this approach is that the original program code is changed by the preprocessor, i.e., line numbers of compiler errors do not actually fit the line numbers of the program. The same problem arises with debugging or runtime exceptions.
• Metaprogramming: According to Templ metaprogramming refers to “programming at the level of program interpretation, or in other words, to extending the interpreter of a given programming language in an application-specific way. Traditionally, this concept is available only in dynamically typed and interpreted languages” [Templ94].

2. How many different kinds of assertions the language supports

Java

Support for assertions

Java, when developed initially, did not have any built-in support for assertions. This was probably because it's exception handling feature was very powerful. However the (beta) release 1.4 of Java 2 included a built-in assertion facility. An assertion is a statement in the JavaTM programming language that enables you to test your assumptions about your program

Python

Support for Assertions

Assertions in Python are built into the language and are implemented using the assert statement. The syntax for the assert statement is as follows:

assert_stmt::="assert" expression ["," expression]

The second expression is optional. Please refer to Python Documentation for more information on the assert statement syntax.

Types of Assertions

Runtime Assertions

The example below checks whether a value of a variable is always positive using assertions in Python.

x=5
assert x>0 #will be true as long as x>0

An AssertionError is raised whenever an assertions fails and usually terminates the program.

The assert statement can also be used for type checking since python is a dynamically typed language. This is useful in making sure the type of the arguments to an function are correct.

from types import *

def foo(x,name)
 assert type(x) is IntType, "id is not an integer: %s" % `id`
 assert type(name) is StringType, "name is not String: %s" % `name`

The string expression at the end of the assert statement is printed out only if the assertion fails.

Assertions during Developmental Cycles

Assertions in python are executed only in debug mode and can be disabled by passing the "-O" option to the python interpreter. This allows the developer to turn off assertions in the release builds to avoid unnecessary performance slowdowns.

Assertions in Design by Contract

Python does not have native support for assertions in Design by Contract(DBC). However, there are third party libraries like PyDBC or Contracts for Python which add support for DBC.

References

• [1] Wikipedia Page Assertions
• [2] Templ J.: Metaprogramming in Oberon, ETH Dissertation No. 10655, Zurich, 1994