[Numpy-discussion] floats coerced to string with "{:f}".format() ?

[Robert Kern]

On Thu, Jun 6, 2013 at 8:48 PM, Maccarthy, Jonathan K <jkmacc at lanl.gov> wrote:
> Hi everyone,
>
> I've looked in the mailing list archives and with the googles, but haven't yet found any hints with this question...
>
> I have a float field in a NumPy record that looks like it's being substituted as a string in the Python "{:f}".format() mini-language, thus throwing an error:
>
>
> In [1]: tmp = np.rec.array([('XYZZ', 2001123, -23.82396)], dtype=np.dtype([('sta', '|S6'), ('ondate', '<i8'), ('lat', '<f4')]))[0]
>
> In [2]: type(tmp)
> Out[3]: numpy.core.records.record
>
> In [3]: tmp
> Out[3]: ('XYZZ', 2001123, -23.823917388916016)
>
> In [4]: tmp.sta, tmp.ondate, tmp.lat
> Out[4]: ('XYZZ', 2001123, -23.823917)
>
> # strings and integers work
> In [5]: '{0.sta:6.6s} {0.ondate:8d}'.format(tmp)
> Out[5]: 'XYZZ    2001123'
>
> # "lat" is a float, but it seems to be coerced to a string first, and failing
> In [6]: '{0.sta:6.6s} {0.ondate:8d} {0.lat:11.6f}'.format(tmp)
> ---------------------------------------------------------------------------
> ValueError                                Traceback (most recent call last)
> /Users/jkmacc/<ipython-input-312-bff8066cfde8> in <module>()
> ----> 1 '{0.sta:6.6s} {0.ondate:8d} {0.lat:>11.6f}'.format(tmp)
>
> ValueError: Unknown format code 'f' for object of type 'str'
>
> # string formatting for doesn't fail
> In [7]: '{0.sta:6.6s} {0.ondate:8d} {0.lat:>11.6s}'.format(tmp)
> Out[7]: 'XYZZ    2001123      -23.82'
>
>
> This also fails:
>
> In [7]: "{:f}".format(np.array(3.2, dtype='f4'))
> ---------------------------------------------------------------------------
> ValueError                                Traceback (most recent call last)
> /Users/jkmacc/<ipython-input-314-33119128e3e6> in <module>()
> ----> 1 "{:f}".format(np.array(3.2, dtype='f4'))
>
> ValueError: Unknown format code 'f' for object of type 'str'
>
>
>
> Does anyone understand what's happening?

numpy.ndarray does not implement the __format__() method. Thus,
str.format() method falls back to object.__format__(). This is the
exception that object.__format__() raises. Why it says "object of type
'str'" is not clear to me. Similarly, numpy.float32 does not implement
the __format__() method. The string scalar type and the native integer
scalar type (I assume you are on a 64-bit platform, so Python ints are
64-bit to match your 'i8' field) inherit from the corresponding native
Python types, so they inherit their __format__() methods.

--
Robert Kern