ELisp: Property List

By Xah Lee. Date: . Last updated: .

What is Property List

Property List (aka plist) is a List , but to be interpreted as list of pairs, like this:

'(key1 val1 key2 val2 etc)

Property list is not supposed to have duplicate keys, and should always have even length.

Key should be lisp Symbols , value can be any lisp object.

Use of Property List

Property List is used as a simplest form of key/value pairs.

Property list is used extensively in emacs.

The 2 major use of property list are:

Property list isn't a generic data structure. If you have more than ten items, you probably should use Association List or Hash Table .

Get a Key's Value

(plist-get PLIST PROP)
return the value of key PROP from property list PLIST. If key does not exist, return nil. Existence of key is checked with eq. [see ELisp: Equality Test]
(plist-get '(x 1 y 2) 'y) ; 2
(plist-get '(x 1 y 2) 'b) ; nil
similar to plist-get, but compare key using equal.

Add/Modify item

(plist-put PLIST PROP VAL)
add or change the value of key PROP in PLIST to VAL. The new plist is returned. Old plist may or may not be changed. Always use setq on original variable.
Existence of key is checked with eq.
;; create a var xx with value of property list
(setq xx '(a 1 b 2))

;; set value to a existing key
(setq xx (plist-put xx 'b 3))

;; (a 1 b 3)
similar to plist-put, but compare key using equal. [see ELisp: Equality Test]

Check Key Existence

(plist-member PLIST PROP)
Return true if PLIST has the property PROP. The PROP is a symbol. Unlike plist-get, this allows you to distinguish between a missing property and a property with the value nil. The return value is actual the tail of PLIST whose car is PROP. [see ELisp: Cons Pair]
(setq xx '(a 1 b 2))

;; check if a key exist
(plist-member xx 'b)
;; (b 2)


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