control precision for str(obj) output?

[Dan Bishop]

Bo Peng wrote:
> Dear list,
>
> I have enjoyed the convenience to output any object with str(obj) for
a
> while. However, I get long output for things like str([0.0002]) in my

> output (which bothers my users more than me though). I also do not
> understand why the following is happening:
>
>  >>> str([0.0002])
> '[0.00020000000000000001]'
>  >>> str(0.0002)
> '0.0002'

Floats make a distinction betwen repr and str: repr(x) uses 17
significant digits (to ensure that repr(1.0) and repr(1.0+epsilon) are
distinct, while str(x) uses only 12 (to produce more user-friendly
output).

For lists, repr(x) calls repr(x[i]) for each of its elements.  Just
what you'd expect.

But lists, unlike floats, do *not* make a distinction between str and
repr; i.e. str(x) == repr(x).  The main reason for doing it this way
was to avoid confusion in cases like str(["1, 2", 3]): If list.__str__
were implemented like the list_str function below, the result would
look like a list of 3 ints instead of a string and an int.

The unfortunate side effect of this is to make str(list of floats)
ugly, but...

> Is there any way to fix this, other than checking the type of
everything
> and output with %.4f?
 
def list_str(x):
    return '[%s]' % ', '.join(map(str, x))