[Python-ideas] More alternate constructors for builtin type

[Josh Rosenberg]

 The other bytes object constructor I often find myself in need of without
being able to remember how to do it is creating a a length 1 bytes object
from a known ordinal. The "obvious":

someordinal = ...
bytes(someordinal)

creates a zeroed bytes of that length, which is clearly wrong. I eventually
remember that wrapping it in a tuple (or list) before passing to the bytes
constructor works, but it's far from intuitive:

bytes((someordinal,))

Unfortunately, the most obvious name for the alternate constructor to fill
this niche is *also* bytes.fromint, which conflicts with Guido's use case.

On Mon, May 6, 2019 at 2:40 PM Guido van Rossum <guido at python.org> wrote:

> 20-25 years ago this might have been a good idea. Unfortunately there's so
> much code (including well-publicized example code) that I'm not sure it's a
> good use of anyone's time to try and fix this.
>
> Exception: I am often in need of a constructor for a bytes object from an
> integer using the decimal representation, e.g. bytes.fromint(42) == b"42".
> (Especially when migrating code from Python 2, where I've found a lot of
> str(n) that cannot be translated to bytes(n) but must instead be written as
> b"%d" % n, which is ugly and unintuitive when coming from Python 2.)
>
> On Mon, May 6, 2019 at 2:50 AM Serhiy Storchaka <storchaka at gmail.com>
> wrote:
>
>> Constructors for builtin types is too overloaded.
>>
>> For example, int constructor:
>>
>> * Converts a number (with truncation) to an integer.
>> * Parses human readable representation of integer from string or
>> bytes-like object. Optional base can be specified. Note that there is an
>> alternate constructor for converting bytes to int using other way:
>> int.frombytes().
>> * Without arguments returns 0.
>>
>> str constructor:
>>
>> * Converts an object to human-readable representation.
>> * Decodes a bytes-like object using the specified encoding.
>> * Without arguments returns an empty string.
>>
>> bytes constructor:
>>
>> * Converts a bytes-like object to a bytes object.
>> * Creates a bytes object from an iterable if integers.
>> * Encodes a string using the specified encoding. The same as str.encode().
>> * Creates a bytes object of the specified length consisting of zeros.
>> Equals to b'\0' * n.
>>
>> dict constructor:
>>
>> * Creates a dict from a mapping.
>> * Creates a dict from an iterable of key-value pairs.
>> * Without arguments returns an empty dict.
>>
>> The problem of supporting many different types of input is that we can
>> get wrong result instead of error, or that we can get error later, far
>> from the place where we handle input.
>>
>> For example, if our function should accept arbitrary bytes-like object,
>> and we call bytes() on the argument because we need the length and
>> indexing, and we pass an integer instead, we will get an unexpected
>> result. If our function expects a number, and we call int() on the
>> argument, we may prefer to get an error if pass a string.
>>
>> I suggest to add limited versions of constructors as named constructors:
>>
>> * int.parse() -- parses string or bytes to integer. I do not know
>> whether separate int.parsestr() and int.parsebytes() are needed. I think
>> round(), math.trunc(), math.floor() and math.ceil() are enough for lossy
>> converting numbers to integers. operator.index() should be used for
>> lossless conversion.
>> * bytes.frombuffer() -- accepts only bytes-like objects.
>> * bytes.fromvalues() -- accepts only an iterable if integers.
>> * dict.frommapping() -- accepts only mapping, but not key-value pairs.
>> Uses __iter__() instead of keys() for iterating keys, and can take an
>> optional iterable of keys. Equals to {k: m[k] for k in m} or {k: m[k]
>> for k in keys}.
>> * dict.fromitems() -- accepts only key-value pairs. Equals to {k: v for
>> k, v in iterable}.
>>
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>
>
> --
> --Guido van Rossum (python.org/~guido)
> *Pronouns: he/him/his **(why is my pronoun here?)*
> <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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