Chances are you have run through this error when you tried using a list as a dictionary key or added it to a set, in Python. It looks confusing at first, but it isn’t as complicated as it might appear. Today we will understand why this error occurs, and how to fix it with ease.<\/p>\n\n\n\n
In Python, dictionary keys and set elements must be hashable. That means:<\/p>\n\n\n\n
A list can be changed at any time, which means its value isn\u2019t stable. Because of that, Python doesn\u2019t allow lists to be used as dictionary keys or set elements. It needs something that stays the same, like a tuple.<\/p>\n\n\n\n
Here\u2019s what causes the error:<\/strong><\/p>\n\n\n\n Output:<\/strong><\/p>\n\n\n\n Or maybe:<\/strong><\/p>\n\n\n\n If your list is just a collection of values and doesn\u2019t need to change, convert it to a tuple:<\/p>\n\n\n\n Tuples are immutable and hashable if all their elements are hashable.<\/p>\n\n\n\n Never use mutable types (like lists or dicts) as keys in dictionaries or elements in sets.<\/p>\n\n\n\n Bad:<\/strong><\/p>\n\n\n\n Good:<\/strong><\/p>\n\n\n\n If your data is inherently set-like but needs to be hashable, use frozenset:<\/p>\n\n\n\n You can also define your own class with custom __hash__ and __eq__ if advanced control is needed.<\/p>\n\n\n\n Ask yourself: do I really need to use a list here?<\/strong><\/p>\n\n\n\n To put it simply, you get this error because lists can change, and Python only allows unchangeable (hashable) types as dictionary keys or set elements. If you need to use list-like data in those places, try using a tuple or frozenset instead. And if you\u2019re unsure about the best structure for your data, it might help to hire Python developers<\/a> who can guide you in building a cleaner, more reliable solution.<\/p>\n","protected":false},"excerpt":{"rendered":" Chances are you have run through this error when you tried using a list as a dictionary key or added it to a set, in Python. It looks confusing at first, but it isn’t as complicated as it might appear. Today we will understand why this error occurs, and how to fix it with ease. […]<\/p>\n","protected":false},"author":2,"featured_media":2407,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[163,3],"tags":[],"class_list":["post-2406","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-python","category-web"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/posts\/2406","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/comments?post=2406"}],"version-history":[{"count":2,"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/posts\/2406\/revisions"}],"predecessor-version":[{"id":2410,"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/posts\/2406\/revisions\/2410"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/media\/2407"}],"wp:attachment":[{"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/media?parent=2406"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/categories?post=2406"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.cmarix.com\/qanda\/wp-json\/wp\/v2\/tags?post=2406"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}my_dict = {[1, 2, 3]: \"numbers\"}<\/code><\/pre>\n\n\n\nTypeError: unhashable type: 'list'<\/code><\/pre>\n\n\n\nmy_set = set()\nmy_set.add([1, 2, 3])<\/code><\/pre>\n\n\n\nSteps to Resolve the Issue<\/h2>\n\n\n\n
Step 1: Use a Tuple Instead of a List<\/h3>\n\n\n\n
my_dict = {(1, 2, 3): \"numbers\"} # \u2705 No error\n\nmy_set = set()\nmy_set.add((1, 2, 3)) # \u2705 Works<\/code><\/pre>\n\n\n\nStep 2: Avoid Mutable Keys<\/h3>\n\n\n\n
my_dict = {[1, 2]: \"value\"}<\/code><\/pre>\n\n\n\nmy_dict = {tuple([1, 2]): \"value\"}<\/code><\/pre>\n\n\n\nStep 3: If You Need Mutability, Use frozenset or Custom Classes<\/h3>\n\n\n\n
my_set = set()\nmy_set.add(frozenset([1, 2, 3])) # \u2705 Works<\/code><\/pre>\n\n\n\nStep 4: Revisit Your Data Structure Design<\/h3>\n\n\n\n
\n
To Conclude<\/h2>\n\n\n\n