Python 3, an Illustrated Tour Transcripts
Chapter: Language syntax
Lecture: Unpacking Generalizations
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In this video, we're going to talk about additional unpacking generalizations. This came out in pep 448 which is in Python 3.5.
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The pep reads extend usages of the * iterable unpacking operator and ** dictionary unpacking operators to allow unpacking
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in more positions an arbitrary number of times. Let's look at some examples here. If I had a dictionary and I wanted to merge it with another one
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in Python 2 and prior to Python 3.5 I would have to do something like this. I would create a dictionary here called thing_colors
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which is mapping some object to the color. So apple is red, pumpkin is orange. And if I wanted to merge this into a new dictionary with more colors
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one way I could do it is create the new dictionary here called more_colors and then I could say update more colors with thing_colors
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that would insert thing_colors into more colors. And then I could update more colors with other values there,
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note that in this case apple I'm overriding it with green. This pep allows us to do something like this
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where I say thing_colors is equal to apples is red and pumpkin is orange and more_colors is equal to,
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and then I make a literal dictionary but inside of that I put ** thing_colors in there.
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What that does is it unpacks the keys and values from that dictionary and copies them into more_colors.
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After that, I'm going to insert bike is blue and apple is green, note that apple here is a repeated key
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and so because this apple comes after, where after means further to the right in my literal dictionary here, then this apple mapping to green
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will overwrite the apple that is red. But note that in more colors here, I have pumpkin as orange in there.
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Also note that Python 3.6 the order of dictionary insertion is remembered apple was the first key that I put in followed by pumpkin
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and note that in more colors apple is first, pumpkin is second and then bike is third,
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because this key was already in there, it stayed in the first position, even though the value is updated.
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So an unpack can be in any location in the dictionary in the previous example, it was at the very start
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but it can be at the end or you can have actually multiple unpacks in a dictionary Here I say bike is blue, apple is green
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and then we're going to unpack thing_colors in there, in this case the apple that's red from there overrides the apple that is green.
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But note that bike and apple are in first and second position because they were in the original dictionary. Another nice feature of this change is
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it allows us to have multiple ** operations in function calls here. Note that here I'm calling this function print args,
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which takes variable number of keyword arguments and I'm saying I want to pass in thing_colors unpacked and also more_colors unpacked.
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This one didn't work prior to Python 3.5, but in Python 3.5 and above you can do syntax like this
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and note that I'm putting in another named parameter here with a value in between those.
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One thing to be aware of though, is that you can't repeat names in a call. So this might be a gotcha here.
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Here I've got thing colors and I've got apple is red here, I've also got more colors apple is green
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and if I call this with thing_colors and more_colors with these repeated keys, I get a type error. In the prior example on the previous slide,
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I didn't repeat that apple key and so I didn't get this error. So this is something you might need to be aware of if you're repeating those.
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Simple way to get around that is just to merge those dictionaries before calling them in there, that's something that might happen.
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So that's the ** operation that allows us to unpack into dictionaries and also use unpacking and function calls in more than one place if we want to.
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Let's look at the single star, single star allows us to create tuples. So here I've got name is equal to Matt.
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Remember that the string Matt is a sequence of characters here. And if I say *name, with a comma at the end here, it's going to give me back a tuple.
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Note that I'm not doing this extended unpack operation, there's no assignment here, so this is slightly different than the prior examples
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where I would get back a list, but this is saying I want you to take what's in the sequence and stick it into a tuple.
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This is also a case where you might need a comma if I have name is equal to Matt and I just say *name on itself,
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I get this error here, syntax error, can't use a star expression here. Here, I've got some examples of various ways we can use star expressions
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and know that by default, these return a tuple and I can actually string these together by putting a comma
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and use multiple of them and here I have a tuple, note that there's parentheses here with all of the characters from name and last in there,
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if I wanted to put those into a list, I just put square brackets around those and that puts it into a list.
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Similarly, I can put it into a set, note that a set is not ordered, so the order comes out a little different than what the order is in name and last.
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Here I've got a little function called summer that just takes variable arguments and variable keyword arguments
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and it's going to sum all the arguments that are passed in in positional locations and it's also going to sum all the values
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so it's not doing very much, it just accepts arbitrary number of arguments and sums them if they are positional or values in keyword arguments,
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let's look at some examples of calling this, I can call it with 1, 2, 3, any number of positional arguments and it gives me 6.
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I can also call it with star list here and then a three following that, and that's fine, Python 3.5 and above doesn't complain about that.
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Note that I can also have multiple stars here and this can be a list here, it can be any sequence here, and I can have
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however many stars I want to in that, if I want to, and here I'm calling it with a star, a positional parameter
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unpacking a dictionary here, a keyword parameter here and another unpacked dictionary over here and that works as well.
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Little bit of the difference between this syntax and the extended unpacking that we showed in another video,
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here I've got name is equal to the string Matt. I can do the extended unpacking by doing an assignment here on the left-hand side
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by saying *letters, I need the comma at the end there is equal to name that makes a new variable called letters
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and it is a list with the individual characters in it. Note that in this unpacking generalization,
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which is different than the extended unpacking defined in this pep here, I am not putting this on the left-hand side,
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I need to put it on the right hand-side, I put a star in front of it, a comma at the end
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and I assign it to a variable and it gives me back a tuple here. So a little bit of a difference between those two,
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if you need to create a list you can use this extended unpacking. Note that if I simply put square brackets around that
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I will also get a list on the second example. This video talked about additional unpacking generalizations, which is pretty cool.
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If you want to create dictionaries from other dictionaries or merge dictionaries or create tuples or lists or sets from sequences,
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you can easily do this with this new syntax. You can also pass in multiple unpacking operations into function calls, which is cool as well.