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

[Maccarthy, Jonathan K]

Ah, so np.int64 and np.str inherit the native Python __format__(), but np.float32/64 doesn't get __builtin__.float.__format__().  That's not intuitive, but I see now why this works:

In [8]: '{:6.6s} {:8d} {:11.6f}'.format(tmp.sta, tmp.ondate, float(tmp.lat))
Out[8]: 'XYZZ    2001123  -23.820000'

Thanks!

-Jon

On Jun 6, 2013, at 1:56 PM, Robert Kern wrote:

> 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
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