Python's comma equals ,= operator?

Key Takeaways

what's up everyone james murphy here from m coding back with another video on python weirdness do you know what the comma equals operator does in python if you know the answer to this trick question let me know down below in the comments otherwise here are some choices in this situation x is one y is the list containing two and we're looking at x comma equals y afterwards we're printing out the value of x so is x the integer one is x the integer two is x the singleton list containing one is x the tuple containing one and list of two or is this just a syntax error pause the video now if you need some time to think about it all right done thinking about it so do you know what the comma equals operator does let's find out by running it x gets the value 2. did you get it right well if you're like me then you probably thought this was just a syntax error when i first saw this in a real code base i thought it was a syntax error and maybe that i accidentally put the comma in myself well that's not true as you can see this does something so if you're not sure about why this printed out too let's look at a simpler example but first let me give you a hint i'll run my code formatter on it did you see what happened here let me do it again ready format it moved the comma do you know the answer now if not let's just go through it here is a simpler example suppose we have a function you know do something and x is 1 and y is 2 then we can say x comma y equals y comma x what this syntax does is it will swap the values of x and y it's kind of doing a little bit of pattern matching let me put in some unnecessary parentheses here it's kind of saying look at this pattern on the left x comma y and try to extract out values for x and y based off of what's on the right hand side where you see y comma x in this case it kind of makes sense that it would swap the values of x and y because i would say well matching the pattern x should be the thing inside the tuple which is at the zeroth index so that's y and y should be the thing which is inside the tuple at the first index which is x so afterwards x is equal to what y was and y is equal to what x was well this would work just as well if this was a list or any other iterable with two elements in it and here's where we see our previous example x comma equals y if you have a list of just one thing or a tuple of just one thing the syntax looks a little different so suppose x was like this a tuple of one thing in in python gives you the following syntax you need to have this extra trailing comma after the single element in the tuple because otherwise it just looks like you parenthesized x and we don't want parentheses to be creating tuples unless you really meant to so in this case if you want the left hand side to be a tuple you need to put that comma in but these parentheses are still optional so now do you see why x comma equals something gives you the value that it did before well our previous case looked like this y was 2 and we just said x comma equals y so now y here is just like we're putting two in so it's saying pattern match this tuple of one thing with this list of one thing that's why x gets the value two at the end of it make sense now is there any real case where you might want to use this syntax admittedly if you're using this syntax you might be in a gray area at the very least absolutely do not format it this way put the spaces in the right place otherwise you're just going to confuse people so i have come up with one case where it might actually you know makes sense to have something like this let's suppose that i have two sets and i know that their intersection has only one element so just for demonstration purposes let's say that we have a list of primes you know 2 3 5 7 11. and we have a list of evens which is 2 4 6 8 10. let me actually make these sets not lists now i know that the set of evens and the set of primes only has one thing in common and that's two so i could say x comma equals with the correct spacing of course primes dot intersect intersection with the evens and then print x so in this case if i call do something now we see that again it prints out two so that's one case where if you know ahead of time that there's only one thing in the right hand side then you might actually want to use this comma equal syntax instead of having a temporary variable you know intersection equals this and then x equals intersection dot pop to get the one element out there you have it that's the comma equals operator in python hey thanks for watching again if you like the video remember to like comment and subscribe also i offer online python courses so if you would like to learn python from me then go ahead and drop your contact info at the link in the description and i'll let you know when online python courses are available