Round default argument for "ndigits"

[jbvsmo]

BPO	19933
Nosy	@terryjreedy, @mdickinson, @bitdancer, @jbvsmo, @vajrasky
Files	fix_round_with_zero_ndigits.patch fix_doc_round_ndigits.patch fix_doc_round_ndigits_v2.patch fix_doc_ndigits_round_and_add_None_ndigits.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2015-04-15.20:16:44.830>
created_at = <Date 2013-12-09.04:24:20.461>
labels = ['interpreter-core', 'docs']
title = 'Round default argument for "ndigits"'
updated_at = <Date 2015-04-15.20:16:44.828>
user = 'https://github.com/jbvsmo'

bugs.python.org fields:

activity = <Date 2015-04-15.20:16:44.828>
actor = 'steve.dower'
assignee = 'docs@python'
closed = True
closed_date = <Date 2015-04-15.20:16:44.830>
closer = 'steve.dower'
components = ['Documentation', 'Interpreter Core']
creation = <Date 2013-12-09.04:24:20.461>
creator = 'JBernardo'
dependencies = []
files = ['33051', '33054', '33060', '33074']
hgrepos = []
issue_num = 19933
keywords = ['patch']
message_count = 17.0
messages = ['205647', '205650', '205651', '205652', '205653', '205682', '205683', '205695', '205699', '205703', '205704', '205706', '205708', '205729', '205770', '206162', '241152']
nosy_count = 7.0
nosy_names = ['terry.reedy', 'mark.dickinson', 'r.david.murray', 'docs@python', 'python-dev', 'JBernardo', 'vajrasky']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue19933'
versions = ['Python 3.5']

[jbvsmo]

From the docs for built-in function "round":
"If ndigits is omitted, it defaults to zero"
(http://docs.python.org/3/library/functions.html#round)

But, the only way to get an integer from round is by not having the second argument (ndigits):

    >>> round(3.5)
    4
    >>> round(3.5, 1)
    3.5
    >>> round(3.5, 0)
    4.0
    >>> round(3.5, -1)
    0.0
    >>> round(3.5, None)
    Traceback (most recent call last):
      File "<pyshell#6>", line 1, in <module>
        round(3.5, None)
    TypeError: 'NoneType' object cannot be interpreted as an integer

Either the docs are wrong or the behavior is wrong. I think it's easier to fix the former...

But also there should be a way to make round return an integer (e.g. passing None as 2nd argument)

[vajrasky]

Here is the preliminary patch.

After patch:

round(1.23, 0) => 1 not 1.0

round(4.67, 0) => 5 not 5.0

[mdickinson]

After patch:
round(1.23, 0) => 1 not 1.0
round(4.67, 0) => 5 not 5.0

Please no! Two-argument round should continue to return a float in all cases.

The docs should be fixed.

[mdickinson]

But also there should be a way to make round return an integer

I don't understand. There's already a way to make round return an integer: don't pass a second argument.

[vajrasky]

Okay, here is the patch to fix the doc.

[mdickinson]

Thanks. It's inaccurate to say that a float is returned in general, though: for most builtin numeric types, what's returned has the same type as its input. So rounding a Decimal to two places gives a Decimal on output, etc. (That's already explained in the next paragraph.)

[mdickinson]

How about just removing the mention of 'defaults to zero', and say something like: "if ndigits is omitted, returns the nearest int to its input"

[jbvsmo]

I don't understand. There's already a way to make round return an integer: don't pass a second argument.

If this function were to be written in Python, it would be something like:

    def round(number, ndigits=0):
        ...

or

    def round(number, ndigits=None):
        ...

But in C you can forge the signature to whatever you want and parse the arguments accordingly. In Python there's always a way to get the default behavior by passing the default argument, but in C it may not exist (in this case PyObject *o_ndigits = NULL;)

So, I propose the default value being None, so this behavior can be achieved using a second argument.

[vajrasky]

Here is the updated doc fix.

Anyway, why not round(1.2) -> 1.0 in the first place? Just curious.

[jbvsmo]

Anyway, why not round(1.2) -> 1.0 in the first place? Just curious.

It was the behavior on Python 2.x, but somehow when they changed the rounding method to nearest even number this happened... I think it's too late to change back the return type.

[bitdancer]

Do you have any real-world motivating use case for None? Not just theoretical consistency with what a Python version of the function would look like. (I'm not saying we shouldn't consider supporting None as a low priority change, I'm just trying to figure out where you'd ever need it in the real world.)

[jbvsmo]

Not really. Just consistency:

For the same reason

' foo '.strip(None)

works... To avoid special casing the function call when you already have a variable to hold the argument.