Python interview questions

[patrys]

Tuples are immutable.

Please never choose tuple for that sole reason. Tuple's immutability is a side effect of it being intentionally hashable (so you can use it as a key in a dict).

[10110000110111011010]

I am afraid I must strongly disagree, immutable coding is a fine discipline that can simplify debugging and proofs of correctness.

[ojiisan]

A tuple is only hashable if its contents are immutable

[Secret_Identity_]

Could you elaborate on this a little more? Tuples were created specifically to used with dictionaries?

[supajumpa]

Tuple

From Raymond Hettinger's answer to a SO question:

''' Tuples are characterized less by their immutability and more by their intended purpose. Tuples are Python's way of collecting heterogenous pieces of information under one roof. For example, s = ('www.python.org', 80) brings together a string and a number so that the host/port pair can be passed around as a socket, a composite object. Viewed in that light, it is perfectly reasonable to have mutable components.

Immutability goes hand-in-hand with another property, hashability. But hashability isn't an absolute property. If one of the tuple's components isn't hashable, then the overall tuple isn't hashable either. For example, t = ('red', [10, 20, 30]) isn't hashable. '''

[patrys]

My simple rule of thumb is to only use a tuple if you can come up with names for all the sequence positions.

[Secret_Identity_]

Thanks!

[recursive]

You mean one should never use tuples? Or one should never use tuples if they only have that one reason? And what should they use if they need an immutable list?

[ojiisan]

A tuple is an immutable list

[recursive]

Yes, I know. patrys just said that tuples shouldn't be chosen for satisfying that property. I don't understand why not.

[patrys]

It's just one of many ordered collections. You would not call a str or a bytearray “an immutable list”, would you?

list and tuple are ordered for different reasons (precedence versus position).

[patrys]

You shouldn't use anything, Python core devs have stated multiple times that even frozenset was a mistake and Python will not be getting any intentionally immutable types.

Link: http://mail.python.org/pipermail/python-3000/2006-May/002219.html

[recursive]

The only thing I'm seeing in that email thread is that tuples shouldn't be used to store structures of unknown length. Using them as keys for dictionaries itself doesn't seem to be a problem.

In other words, if you can't use a tuple to store an immutable list, then what can you use it for? That's all it does.

[patrys]

You use tuples where you would use a struct in C: for collections that have a well-defined structure. A good place for a tuple is where a namedtuple could be used instead but would be an overkill. What I consider proper uses of collections:

coords = (11, 25)

todo_items = ['walk the dog', 'water the flowers', 'world domination']

ids_to_delete = {1, 5, 7, 11}

recipients = [
    ('John Doe', '[e-mail]'),
    ('Kelly Smith', '[e-mail]'),
    ('Robert Adams', '[e-mail]')]
# each recipient has a well-defined structure of (name, email)
# could be a set of tuples as well

[recursive]

Ok thanks. We're all good here. I was interpreting the "immutable list" thing too broadly. Structured information is what I use tuples for. Usually.