Decimal constructor to accept float

[rhettinger]

BPO	8257
Nosy	@rhettinger, @mdickinson, @ericvsmith, @merwok
Files	deci_float_constructor.diff: decimal_from_float.diff

^{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 2010-04-06.14:34:03.814>
created_at = <Date 2010-03-28.23:12:19.560>
labels = ['easy', 'type-feature']
title = 'Decimal constructor to accept float'
updated_at = <Date 2010-04-06.14:34:03.812>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2010-04-06.14:34:03.812>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2010-04-06.14:34:03.814>
closer = 'mark.dickinson'
components = []
creation = <Date 2010-03-28.23:12:19.560>
creator = 'rhettinger'
dependencies = []
files = ['16683']
hgrepos = []
issue_num = 8257
keywords = ['patch', 'easy']
message_count = 9.0
messages = ['101882', '101898', '101899', '101905', '102149', '102181', '102182', '102183', '102468']
nosy_count = 4.0
nosy_names = ['rhettinger', 'mark.dickinson', 'eric.smith', 'eric.araujo']
pr_nums = []
priority = 'high'
resolution = 'accepted'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue8257'
versions = ['Python 2.7', 'Python 3.2']

[rhettinger]

Per discussion on python-dev, let Decimal(x) accept floats as a possible input, making Decimal.from_float unnecessary.

See attached patch.

[merwok]

Will Decimal.from_float be deprecated and eventually removed?

Could you provide a link to said discussion thread for us outsiders? :)

Regards

[rhettinger]

Éric, there is no place to deprecate Decimal.from_float(). It will be redundant but there is no need to break working code. Also, it has the nice property of being explicit about what it is doing.

Link to my proposal:
http://mail.python.org/pipermail/python-dev/2010-March/098699.html

Guido's "If you really want this I won't stop you":
http://mail.python.org/pipermail/python-dev/2010-March/098701.html

[merwok]

(Assuming “no place” means “no reason”) Apart from TOOTDI, there is
indeed none.

Regards

[mdickinson]

Raymond, do you want this to go into 2.7 as well? I'm assuming that we're not going to allow mixed-type float+decimal operations in 2.7.

Also, if we're doing this, it seems to me that all the reasons you give for the Decimal constructor accepting floats also apply to the Fraction constructor.

The patch looks fine to me. The new testcase is specific to IEEE 754 binary64 floating-point format, so I'll slap a 'requires_IEEE_754' decorator on it (copying the definition from test_math.py), if that's okay.

[rhettinger]

Added the requires decorator as requested.
Committed in r79602. Will backport to 2.7 this weekend.

And yes, I agree that Fraction(somedecimal) should work too. It would be weird to have Fraction("1.1") work but not Fraction(Decimal("1.1")).

Mark, I do think we should have decimal+float-->float in 2.7 also. I don't see any reason to have the two versions diverge on this issue.

[mdickinson]

Mark, I do think we should have decimal+float-->float in 2.7 also.

That's fine with me in principle. But isn't 2.7 in feature freeze from tomorrow?

[mdickinson]

And yes, I agree that Fraction(somedecimal) should work too.

What about Fraction(1.1)?

[mdickinson]

The decimal changes were implemented by Raymond in r79609 (trunk) and r79602 (py3k). For the fractions module changes, see bpo-8294.