Extract Variable

Before you begin: Variables, arithmetic, and comparisons.

Extract Variable gives part of an expression a name. The new variable explains what a value represents instead of making every reader mentally interpret the arithmetic. 1

Original version: decode the units

A video upload must be no longer than five minutes. Inputs are a nonnegative minute count and a second count between 0 and 59.

public static class Clips
{
    public static bool Fits(int minutes, int seconds)
    {
        return minutes * 60 + seconds <= 5 * 60;
    }
}

The expression converts the clip length to seconds and compares it with the five-minute limit. That meaning is present, but only in the arithmetic.

def fits(minutes, seconds):
    return minutes * 60 + seconds <= 5 * 60

The expression converts the clip length to seconds and compares it with the five-minute limit. A reader has to identify both quantities before understanding the comparison.

Refactored version: name the quantities

public static class Clips
{
    public static bool Fits(int minutes, int seconds)
    {
        int durationSeconds = minutes * 60 + seconds;
        int maximumSeconds = 5 * 60;
        return durationSeconds <= maximumSeconds;
    }
}

durationSeconds and maximumSeconds make the comparison explicit. Fits(5, 0) remains true, and Fits(5, 1) remains false.

def fits(minutes, seconds):
    duration_seconds = minutes * 60 + seconds
    maximum_seconds = 5 * 60
    return duration_seconds <= maximum_seconds

The new names explain both the values and their units. fits(5, 0) remains true, and fits(5, 1) remains false.

Preserve when an expression runs

Here all inputs are ordinary local numbers and both calculations always ran in the original version. Naming them does not change which work occurs.

That is not true of every extraction. In isReady && ExpensiveCheck(), the second operation runs only when isReady is true. Calculating ExpensiveCheck() in a variable before the condition would make it run unconditionally. Exceptions, side effects, and cost could all change.

Choose a scope that preserves the original evaluation. Prefer names that explain the domain or units, not temp1 and temp2. Conversely, a long list of variables for obvious subexpressions can bury a simple calculation. The useful variable answers a question the expression left implicit.

Sources

Martin Fowler — Extract Variable.
Microsoft — Boolean logical operators, including conditional evaluation of && and ||.