A Crazy Python Dictionary Expression ?!

Key Takeaways

hey and welcome to the next video in the python tricks Series so python tricks is a new book that I'm working on and it's kind of like an a buffet of awesome python features so um the goal with the book is to give you you know a really quick overview with a simple code example and kind of a guided walkr through one really cool aspect of python that you might not have heard about and this could be you know a little known syntactical feature a little known feature in the python standard library and it's it's all very practical stuff that's going to help you um get better in your python coding and so today um I want to cover a particular python trick that um I guess it's a little bit more quirky but uh it'll still lead to some really interesting considerations and uh so what I want to talk about today is a really crazy dictionary expression and so I'm going to move this out of the way here and uh yeah so this dictionary expression is uh pretty surprising at least it was really surprising to me when I saw it for the first time and so let me just type this out here real quick so basically what we're doing here is we're putting in creating a new dictionary and we're putting in a bunch of keys and um uh keep like keep those keys in mind and just kind of look at those keys and and maybe you'll know what's going to happen when I hit return and evaluate this expression so um I'm just going to wait for a second like think about this what what do you think this is going to what do you think is going to happen when we evaluate this expression what do you think the outcome of this would be like actually pause the video and think about this for like a minute or two and then come back to it okay ready I'm going to run this now boom okay so the result we're getting here is a dictionary which just one key it has just one key true and the value is maybe and actually when we look at the original expression here that's kind of surprising in my books right uh because well the true key well that that was the key we put here and actually the value was supposed to be yes and then that no and one doesn't show up at all and here we have uh one 1.0 the float value uh as the key and and this may be value for that key now actually shows up here so this this seems like a complete mess and uh this seems you know this is a really surprising result so I want to untangle that now because it's going to show you um I think it's going to help you to better understanding or deeper understanding of uh the python interpreter and how python python works like certainly helped me with that well the first thing we should do here is try to break down how python processes our dictionary expression here I think that gives us a hint right because it kind of makes us realize that well as we're assigning these Keys they're we're overriding them right like the key one the integer one overrides the key true and the integer um 1 Point uh the float 1.0 overrides the other key and well I mean it doesn't really override the key but it overrides the value right so we we end up with like it seems to be taking the keyy from the first time we set this particular key and it seems to take the value from the last time we set this particular key I mean at this point we don't really know like why are these considered all the same key but it kind of gives us a hunch right like what what could be wrong here like let's play python detective here so if we're actually try and like uh validate this assumption or test this assumption let's just compare all of the keys that we're using here and actually python tells us that true one and 1.0 they're all considered um equal they're all considered to be the same key and so that's the case because python actually treats booleans as a subtype of the inte integer type and you can dig that up in the python documentation here in the uh standard type hierarchy so um when we search for Bo here you can see here that the Boolean type is a subtype of the integer type and Boolean values behave like the values zero and one respectively in almost all contexts with the or the exception being that when converted to a string the strings false or true are returned respectively and that's our explanation right that pretty much explains what's going on here it doesn't really explain why we're not ending up with the key uh 1.0 at the end but it kind of explains how these keys are getting overridden so what we can see here that as we're building up this dictionary we're overriding the value for that key and because you might be wondering why we're still ending up with um true as the value or like the name for that key that is the case because if if the keys are considered equal python doesn't actually have to update the object right if they're considered equal it can just keep the initial object and update the value and so what you end up with if we play through this again let's play through this again let's actually let's uh let's start with a new uh with a new dictionary here so if we play through this again we're going to say okay Y is true equals yes and we print out y's okay that makes sense right and now we're going to say Y is 1 equals no all right what do we expect now well we expect the key to remain the same and it to be overridden with um with a new value and now if we uh set the key one to maybe then again we expect to end up with the result that we've gotten out of this dictionary expression very early on so this seems to confirm our our Theory here all right and that dispels most of this magic here now there might be you know at the back of your head you might be thinking oh aren't python dictionaries hash Maps or something is that like is that what a hash Collis is um sort of uh and actually I've got a lot more to say on this topic to kind of really dive down into the hashmap stuff and really uh untangle where that that comes into place so you can either check out the book for that um or you can read a blog post that I wrote about this earlier so uh that that there are going to be some pointers for you to dive more deeply into this and they're going to be linked in the description for the video and again if you enjoyed this video or this video series then please check out my book uh called python tricks and I hope I see you soon