Quality Code: Software Testing Principles, Practices, and Patterns

Locally Redefine

Some languages let you redefine any variable or function within a scope and only for the duration of that scope. Let’s say, for example, that we want to test a Perl subroutine that uses the open function. Consider how you might force an error in a test of the following line of idiomatic Perl.

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open $fh, "<", "input.txt"    or die "Could not open input file.";

The open function returns a nonzero value on success and the Perl undefined value otherwise. The short-circuit evaluation of the or evaluates the open as true on success and therefore skips the die statement. But how would we get open to return undef?

The test in Listing 6-9 shows how to use local to override open for the duration of the test ...