Float formatting with no type code produces incorrect output for exponential notation

[ericvsmith]

BPO	5686
Nosy	@mdickinson, @ericvsmith

^{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 = 'https://github.com/ericvsmith'
closed_at = <Date 2009-04-05.13:26:36.874>
created_at = <Date 2009-04-04.12:20:16.175>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = 'Float formatting with no type code produces incorrect output for exponential notation'
updated_at = <Date 2009-04-05.13:26:36.873>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2009-04-05.13:26:36.873>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2009-04-05.13:26:36.874>
closer = 'eric.smith'
components = ['Interpreter Core']
creation = <Date 2009-04-04.12:20:16.175>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 5686
keywords = []
message_count = 4.0
messages = ['85386', '85387', '85391', '85489']
nosy_count = 2.0
nosy_names = ['mark.dickinson', 'eric.smith']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue5686'
versions = ['Python 2.6', 'Python 3.0', 'Python 3.1', 'Python 2.7']

[ericvsmith]

If no format specifier is supplied to a float, it's supposed to work
like 'g', but with at least one digit after the decimal. This works for
non-exponential notation:

>>> format(1., '')
'1.0'
>>> format(1., 'g')
'1'

But for exponential notation, it does not:
>>> format(1e200, '')
'1e+200'
>>> 
>>> format(1e200, 'g')
'1e+200'

[ericvsmith]

The documentation doesn't agree with the PEP. The docs at
http://docs.python.org/library/string.html#formatstrings just say '' is
the same as 'g'. It does not draw a distinction between exponential and
fixed output.

The PEP has the "at least one digit after the decimal" language. It also
does not draw a distinction between exponential and fixed output.

[mdickinson]

I don't really see this as a bug in the behaviour. I'd always understood
the purpose of the "at least one digit after the decimal" to be to make it
possible to visually distinguish floats and integers, in the same way that
repr and str already do.

>>> repr(1e100)
'1e+100'
>>> repr(1e5)
'100000.0'

If an exponent's present then the string's already clearly not an integer,
so there's no need for the '.0'.

[ericvsmith]

This isn't testing what I thought it was testing. I'd forgotten that
format(x, '') is exactly equivalent to str(x).

The better test is using a format string of '<':
>>> 1e200.__format__('<')
'1.0e+200'
>>> 1e200.__format__('<g')
'1e+200'

So this is a non-issue: the current behavior matches the PEP
description. The online docs could still be tweaked, though.