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par flopar
Publié: 17 juin 2022 (il y a 2 semaines)






Python lists ‘are mutable sequence type’ even with append()

Consider the following code:
def t():
l = []
print ‘l’, l, l.pop()

The output is:
l [] a
l [a] b

The documentation says:

The list type is mutable sequence type, which means that it’s an
object that supports indexing and iteration. The append() method adds
an item to the end of a list.

Also the documentation says:

“Sequence Types: The mutable sequence types are included in the
sequence types (list, tuple, set, and frozenset) for compatibility.
When you write a for loop over a mutable sequence, the loop will
modify the sequence in place.”

Why is l not immutable?


PEP 265 extends the introduction of mutable sequence types to namedtuple, dict, sets, and frozenset too.

it’s an object that supports indexing and iteration.

That’s also explained in Mutable Sequence Types:

You’ll notice that the sequence types behave a little bit differently than the other types you’ve seen so far. For one thing, a mutable sequence type is an object, not a value. If you create an instance of the SequenceType, it has a reference to its own list object; there is no copy of the list.

Implementation details

Why is l not immutable?

Due to the fact that the type of l is [], it is mutable.

Here’s an example of the mutable type:
>>> t = [‘a’,’b’]
>>> t
[‘a’, ‘b’]
>>> t.append(‘c’)
>>> t
[‘a’, ‘b’, ‘c’]
>>> t[0] = ‘x’
>>> t
[‘a’, ‘x’, ‘b’, ‘c’]

Adding the ‘c’ value to the end of the list, while referencing the original l (by indexing), results in the changes are reflected in the original l:
>>> l[0] = ‘y’
>>> l

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« There are more bacteria on a toilet seat than there are human beings on the entire planet, » – Robbin Williams, Wired UK

I remember reading somewhere that this is definitely more true than not; and as anyone that reads this will probably now, no matter how interesting it is to read. I think.

Anyway, what I like is the simple problem: how would we ever find a planet where we can survive without first disinfecting the toilet?

1. Find a planet with a magnetic field which is great and strong enough to break the toilet’s magnetic field.2. Put a 1000 foot diameter hole in the toilet.3. Put a small transmitter in the toilet.4. Place a 100 foot diameter hole in the transmitter.5. Wait.

But what would happen if we weren’t looking to find the toilet? What if we just wanted to stop a certain number of dysentery’s happening in our outback community?

We would have to drill a 100 foot deep hole. At that depth we’d be looking at about 1,000,000 cubic meters of earth (give or take a few liters).

Now, what would we drill in it? Probably look for some of the large underground seas we’ve heard about.

However, it would be big news if, while drilling, you found something. It would be either a large man-made object like the shafts the Romans used, or more likely some ancient temple, ruins, or (most likely)

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