(setf (get symbol indicator &optional default) new-value)

indicator - an object.

default - an object. The default is nil.

value - if the indicated property exists, the object that is its value; otherwise, the specified default.

new-value - an object.

setf of get may be used to associate a new object with an existing indicator already on the symbol's property list, or to create a new assocation if none exists. If there are multiple properties₁ with that property indicator, setf of get associates the new-value with the first such property. When a get form is used as a setf place, any default which is supplied is evaluated according to normal left-to-right evaluation rules, but its value is ignored.

 (defun make-person (first-name last-name)
   (let ((person (gensym "PERSON")))
     (setf (get person 'first-name) first-name)
     (setf (get person 'last-name) last-name)
     person))   MAKE-PERSON
 (defvar *john* (make-person "John" "Dow"))  *JOHN*
 *john*  #:PERSON4603
 (defvar *sally* (make-person "Sally" "Jones"))  *SALLY*
 (get *john* 'first-name)  "John"
 (get *sally* 'last-name)  "Jones"
 (defun marry (man woman married-name)
   (setf (get man 'wife) woman)
   (setf (get woman 'husband) man)
   (setf (get man 'last-name) married-name)
   (setf (get woman 'last-name) married-name)
   married-name)   MARRY
 (marry *john* *sally* "Dow-Jones")  "Dow-Jones"
 (get *john* 'last-name)  "Dow-Jones"
 (get (get *john* 'wife) 'first-name)  "Sally"
 (symbol-plist *john*)
 (WIFE #:PERSON4604 LAST-NAME "Dow-Jones" FIRST-NAME "John")
 (defmacro age (person &optional (default "thirty-something)) 
   `(get ,person 'age ,default))   AGE
 (age *john*)   THIRTY-SOMETHING
 (age *john* 20)  20
 (setf (age *john*) 25)  25
 (age *john*)  25
 (age *john* 20)  25

 (get x y) ==(getf (symbol-plist x) y)

Numbers and characters are not recommended for use as indicators in portable code since get tests with eq rather than eql, and consequently the effect of using such indicators is implementation-dependent.

There is no way using get to distinguish an absent property from one whose value is default. However, see get-properties.