beginner25 minby DevFox

Lists and Tuples

Ordered collections you can change and ones you cannot: indexing, slicing, growing a list, and the aliasing surprise where two names turn out to share one list.

Python

You have four hundred photographs to process, and everything you know so far handles exactly one thing at a time. You could write four hundred variables. Nobody has ever done this twice.

What you need is a single name for many values, which you can add to, look through and hand around as one thing. By the end of this lesson you will have two of them — one you can change and one you cannot — and you will know why the second exists, which is less obvious and more useful than it first appears.

Many values, one name

A list holds values in order. Write it with square brackets:

python

photos = ["dawn.jpg", "tram.jpg", "square.jpg"]
print(len(photos))       # 3

A list can hold anything, including a mixture, though in practice you usually keep one kind of thing in one list:

python

mixed = ["dawn.jpg", 400, 3.75, True, None]

An empty list is where most lists start, because you build them up as you go:

python

processed = []

Getting things out

Positions start at zero, exactly as with text:

python

photos = ["dawn.jpg", "tram.jpg", "square.jpg"]

print(photos[0])     # dawn.jpg
print(photos[2])     # square.jpg
print(photos[-1])    # square.jpg   - last, without counting
print(photos[-2])    # tram.jpg

Asking for a position that does not exist is an error rather than a silent empty answer, which is the behaviour you want:

python

print(photos[9])
# IndexError: list index out of range

Slicing takes a range, on the same "up to but not including" rule as strings:

python

print(photos[0:2])    # ['dawn.jpg', 'tram.jpg']
print(photos[:2])     # the first two
print(photos[1:])     # everything from position 1
print(photos[-2:])    # the last two

A slice always gives you a new list. This matters, and the next section explains why.

Changing a list

Lists are mutable — you can change one after it exists. This is the main difference from text.

python

photos[0] = "sunrise.jpg"        # replace in place

photos.append("bridge.jpg")      # add one to the end
photos.extend(["a.jpg", "b.jpg"])# add several
photos.insert(0, "first.jpg")    # add at a position

photos.remove("tram.jpg")        # remove by value (the first match)
last = photos.pop()              # remove and return the last
second = photos.pop(1)           # remove and return position 1
del photos[0]                    # remove by position
photos.clear()                   # empty it

append is the one you will use most, because building a list by adding to it in a loop is one of the most common shapes in all of programming.

Note what these methods return. append, extend, insert, remove and sort all change the list and hand back None — they are not like string methods, which leave the original alone and return a new value. This produces a specific, memorable mistake:

Bad — assigning the result of a method that changes the list.

python

photos = ["tram.jpg", "dawn.jpg"]
photos = photos.append("bridge.jpg")
print(photos)        # None

Good — calling it and keeping the list.

python

photos = ["tram.jpg", "dawn.jpg"]
photos.append("bridge.jpg")
print(photos)        # ['tram.jpg', 'dawn.jpg', 'bridge.jpg']

The first version does exactly what you wrote: it appends successfully, then throws the list away and puts None under the name instead. The next line that touches photos fails with something about NoneType, several steps from the actual mistake. The rule is worth stating flatly — methods that change a list return None — and it is the opposite of the string rule from two lessons ago.

The aliasing trap, properly

You met this briefly in the lesson on names. Now that you have lists, it is worth doing properly, because it is the single most common source of "I didn't change that" bugs.

originals = ["dawn.jpg", "tram.jpg"]
One list exists, with one label tied to it.
backup = originals
This is where the bug is. It looks like a copy and is not — it ties a second label to the same list. Nothing was duplicated.
backup.append("square.jpg")
The list changes. There is only one list, so it changes for both labels.
print(originals) — three items
You never wrote originals on the line that changed it, and it changed anyway.

Four lines, and the surprise is in the one that does the least

To get an actual copy, ask for one:

Now changing backup leaves originals alone.

Sorting and ordering

Two ways, and the difference is the mutability rule again:

sort() is a method on the list. sorted() is a built-in function that works on anything you can loop over, and always hands back a new list. When in doubt, use sorted() — not changing things unexpectedly is worth the extra characters.

That key argument says what to sort by, and the capital letter surprise is why it exists: sorting text compares character codes, and every capital letter sorts before every lowercase one.

Tuples: a list that cannot change

A tuple is an ordered collection you cannot modify. Round brackets instead of square:

The obvious question is why anyone would want a collection they cannot change.

A list

A sequence that will grow and shrink.

Every position means the same kind of thing, and there could be more or fewer of them tomorrow.

Hand one to a function and that function can modify it.

A tuple

One record, and this is all of it.

(1920, 1080) is a size, not a list that happens to hold two numbers today. The positions mean different things.

Nothing can modify it by accident, and — because it cannot change — it can be a dictionary key, which the next lesson relies on.

The restriction is the feature. Three things follow from it.

Tuples appear whether or not you write them yourself. Returning several values from a function makes one:

That last line is unpacking: pulling a tuple apart into separate names in one step. It works anywhere:

The swap is a small delight the first time you see it. The *rest form collects whatever is left over into a list.

Choosing between them

A workable rule: reach for a list by default, and a tuple when the collection is a fixed record rather than a sequence.

The test that usually settles it: if the positions mean different things — width then height, latitude then longitude — it is a tuple. If every position means the same kind of thing and there could be more or fewer of them, it is a list.

A cheat sheet to keep

Where to go next

You can now hold many values under one name, reach into them, build them up, sort them, and copy them without accidentally sharing them. The None-returning rule and the aliasing trap are the two that will actually catch you, and both are now things you have seen deliberately rather than discovered at midnight.

Next is Dictionaries and Sets, which answer a different question. A list is right when position matters. When you want to look something up by name — this photographer's photo count, this file's size — position is the wrong handle, and a dictionary is the tool.

Before you move on, build a list by appending to it in a loop, then make a copy the wrong way and prove to yourself that both names changed. Then fix it with .copy(). Doing that deliberately takes two minutes and saves you the version of the same bug that takes two hours.