Rounding when converting float tuple to str

[scientist47]

BPO	7010
Nosy	@rhettinger, @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 = None
closed_at = <Date 2009-09-28.09:14:34.492>
created_at = <Date 2009-09-28.07:43:56.917>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = 'Rounding when converting float tuple to str'
updated_at = <Date 2009-09-28.18:14:07.067>
user = 'https://bugs.python.org/scientist47'

bugs.python.org fields:

activity = <Date 2009-09-28.18:14:07.067>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2009-09-28.09:14:34.492>
closer = 'mark.dickinson'
components = ['Interpreter Core']
creation = <Date 2009-09-28.07:43:56.917>
creator = 'scientist47'
dependencies = []
files = []
hgrepos = []
issue_num = 7010
keywords = []
message_count = 6.0
messages = ['93193', '93195', '93196', '93197', '93202', '93222']
nosy_count = 4.0
nosy_names = ['rhettinger', 'mark.dickinson', 'eric.smith', 'scientist47']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue7010'
versions = ['Python 2.6', 'Python 3.0']

[scientist47]

When a floating point value is stored, the actual value stored is
something at most some small number eps larger or smaller than the
original value. Python knows this, so if it stores 0.1, and then prints
the stored value, it rounds off decimals less significant than eps.
For some reason, if it prints a tuple with the same value, it doesn't do
the rounding properly. This behavior is incorrect, and very annoying for
instance when printing manually entered constants in tuples.

'''Shows that floats in tuples are not rounded like floats.
>>> print(.1)
0.1
>>> print((.1,))
(0.10000000000000001,)
'''
import doctest
doctest.testmod(verbose=True)

[ericvsmith]

What OS, processor, and Python version are you running this code on?

From your example, it's Python 3.x. 3.1 has a completely rewritten
float->decimal conversion system, and I get different results.

>>> print(.1)
0.1
>>> print((.1,))
(0.1,)
>>> 

For 2.6 and 3.0, you'll get the old behavior, and this won't change. Are
you really using Python 3.0 (which is marked in the versions)? If so,
switch to 3.1 and see what you get.

[mdickinson]

This is expected (and correct) behaviour for Python 2.x and 3.0. Note
that the first example calls .1.__str__, while the second calls .1.__repr__.

Python 2.6.2 (r262:71600, Aug 26 2009, 09:40:44)
[GCC 4.2.1 (SUSE Linux)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> str(.1)
'0.1'
>>> repr(.1)
'0.10000000000000001'
>>> str((.1,))
'(0.10000000000000001,)'
>>> tuple(str(x) for x in (.1,))
('0.1',)

[mdickinson]

And as Eric says, you shouldn't see this behaviour in Python 3.1, since
there str(0.1) == repr(0.1) == '0.1'. If you are seeing it in 3.1, then
something interesting's happening: please reopen in that case. :)

[scientist47]

I tried Python 3.1, and it does indeed not have this issue. Thanks for
swift response!

[rhettinger]

You're seeing the difference between str(.1) and repr(.1). When
printing the tuple, you're getting the str of the tuple but just the
repr of its contents ('%.17g'). When printing the number directly,
you're just getting the str of the number which is rounded to fewer
decimal places for display ('%.12g').

This is easily seen with a number like pi:

>>> from math import *
>>> print(pi)
3.14159265359
>>> print((pi,))
(3.141592653589793,)
>>> repr(pi)
'3.141592653589793'
>>> str(pi)
'3.14159265359'

In Py3.1, the repr computation was changed to show the shortest string
the where float(x)==x. In the case of .1, the repr and str value happen
to be the same (that is why Eric sees no difference in 3.1).