Change default float print format

Question

Hi, I've some lists and more complex structures containing floats. When printing them, I see the floats with a lot of decimal digits, but when printing, I don't need all of them. So I would like to define a custom format (e.g. 2 or 3 decimals) when floats are printed.

I need to use floats and not Decimal. Also, I'm not allowed to truncate/round floats.

Is there a way to change the default behavior?

TIA ~Aki

Answer 1

No, because that would require modifying float.__str__(), but you aren't allowed to monkeypatch C types. Use string interpolation or formatting instead.

Answer 2

>>> a = 0.1
>>> a
0.10000000000000001
>>> print a
0.1
>>> print "%0.3f" % a
0.100
>>>

From the Python docs, repr(a) would give 17 digits (as seen by just typing a at the interactive prompt, but str(a) (automatically performed when you print it) rounds to 12.

Edit: Most basic hack solution... You have to use your own class though, so...yeah.

>>> class myfloat(float):
...     def __str__(self):
...             return "%0.3f" % self.real
>>> b = myfloat(0.1)
>>> print repr(b)
0.10000000000000001
>>> print b
0.100
>>>

Answer 3

You are not aloud to monkeypatch C types, like Igacio said.

However, if you are terribly pressed in doing so and you know some C, you could go modify the Python interpreter source code yourself, then recompile it into a custom solution. Once I modified one of the standard behaviors for lists and it was only a moderate pain.

I suggest you find a better solution, such as just printing the floats with the "%0.2f" printf notation:

for item in mylist:
    print '%0.2f' % item,

or

print " ".join('%0.2f' % item for item in mylist)

Answer 4

Upgrade to Python 3.1. It doesn't use more digits than necessary.

Python 3.1.2 (r312:79147, Apr 15 2010, 15:35:48) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 0.1
0.1