Improve f-strings documentation wrt format specifiers

[Mariatta]

BPO	31487
Nosy	@ericvsmith, @vadmium, @Mariatta
PRs	bpo-31487: Improve f-strings documentation #3604 bpo-31487: Improve f-strings documentation (GH-3604) #3619 bpo-31487: Update F-strings doc example #3627 [3.6] bpo-31487: Update F-strings doc example (GH-3627) #3628

^{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 2017-09-16.19:01:58.324>
created_at = <Date 2017-09-15.17:34:45.675>
labels = ['3.7', 'docs']
title = 'Improve f-strings documentation wrt format specifiers'
updated_at = <Date 2017-09-18.00:14:24.732>
user = 'https://github.com/Mariatta'

bugs.python.org fields:

activity = <Date 2017-09-18.00:14:24.732>
actor = 'martin.panter'
assignee = 'docs@python'
closed = True
closed_date = <Date 2017-09-16.19:01:58.324>
closer = 'Mariatta'
components = ['Documentation']
creation = <Date 2017-09-15.17:34:45.675>
creator = 'Mariatta'
dependencies = []
files = []
hgrepos = []
issue_num = 31487
keywords = ['patch']
message_count = 5.0
messages = ['302279', '302350', '302367', '302368', '302397']
nosy_count = 4.0
nosy_names = ['eric.smith', 'docs@python', 'martin.panter', 'Mariatta']
pr_nums = ['3604', '3619', '3627', '3628']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue31487'
versions = ['Python 3.6', 'Python 3.7']

[Mariatta]

Provide a couple more examples of using format specifiers in f-string.

[Mariatta]

New changeset 46c1052 by Mariatta (Miss Islington (bot)) in branch '3.6':
bpo-31487: Improve f-strings documentation (GH-3604) (GH-3619)
46c1052

[Mariatta]

New changeset 63c591c by Mariatta in branch 'master':
bpo-31487: Update F-strings doc example (GH-3627)
63c591c

[Mariatta]

New changeset 4f6bae9 by Mariatta (Miss Islington (bot)) in branch '3.6':
bpo-31487: Update F-strings doc example (GH-3627) (GH-3628)
4f6bae9

[vadmium]

>> f"{number:#0x}" # using integer format specifier

It’s not clear what your purpose was adding the above line, but the zero flag (0) does nothing because there is no “width” field. I think it could be misleading, because it is actually the “#x” codes that generate the “0x” prefix.

If you want to illustrate a minimum width, I suggest something like

>>> f"{number:#06x}"
'0x0400'

or (if the number is never negative)

>>> f"0x{number:04X}"
'0x0400'

Or if you don’t care about the width:

>>> f"{number:#x}"
'0x400'