Yoda conditions

In programming jargon, Yoda conditions (also called Yoda notation) is a programming style where the two parts of an expression are reversed from the typical order in a conditional statement. A Yoda condition places the constant portion of the expression on the left side of the conditional statement.

Yoda conditions are part of the coding standards for Symfony and WordPress.

Origin
The name for this programming style is derived from the Star Wars character Yoda, who speaks English with a non-standard syntax (e.g., "When 900 years old you reach, look as good you will not." ).  Thomas M. Tuerke claims to have coined the term Yoda notation and first published it online in 2006. According to him, the term Yoda condition was later popularized by Félix Cloutier in 2010.

Example
Usually a conditional statement would be written as:

Yoda conditions describe the same expression, but reversed:

Readability of logically-chained comparisons
Some languages, such as Python, support "chained" comparison operators ("comparators") in their syntax. Thus, the following lines are logically equivalent:

Notice that the second form naturally uses Yoda syntax in the left-hand comparison (3.14 < y). Consider the same line without Yoda syntax:

When handwriting math, many authors prefer the "chained" notation (example, example). When programming in a language that does not literally support the chained notation, the author may prefer the Yoda syntax, as it at least visually evokes the familiar chained notation.

Error detections
Placing the constant value in the expression does not change the behavior of the program (unless the values evaluate to false—see below). In programming languages that use a single equals sign for assignment expressions and not for comparison, a possible mistake is to assign a value unintentionally instead of writing a conditional statement.

Using Yoda conditions:

Since 42 is a constant and cannot be changed, this error will be caught by the compiler.

Avoiding some types of unsafe null behavior
Yoda conditions help with unsafe behavior in some situations.

With Yoda conditions:

Criticism
Yoda conditions are criticized for compromising readability by increasing the cognitive load of reading the code.

Some programming languages (such as Swift, Kotlin and versions of Python below 3.8) do not allow variable assignments within conditionals—for example by requiring that assignments do not return a value, or by defining as part of their grammar the invariant that conditions cannot contain assignment statements—in which case this error is impossible to encounter (that is, it would be detected as a syntax error by the parser prior to a program ever being allowed to enter into runtime). Many compilers produce a warning for code such as  (e.g., the GCC   option warns suggest parentheses around assignment used as truth value), which alerts the programmer to the likely mistake. In dynamic languages like JavaScript, linters such as ESLint can warn on assignment inside a conditional. Python 3.8 introduced assignment expressions, but uses the walrus operator  instead of a regular equal sign (=) to avoid bugs which simply confuse   with.

The advantage of avoiding null behavior can also be considered a disadvantage, as null pointer errors can be hidden and only appear much later in the program.

Another disadvantage appears in C++ when comparing non-basic types as the == is an operator and there may not be a suitable overloaded operator function defined. Example: a Microsoft's  compare against a string literal, written as , does not map to an overload function.