It would be nice to be able to use the Python syntax we already use to format the precision of floating numbers in numpy:
>>> a = np.array([-np.pi, np.pi])
>>> print(f"{a:+.2f}")
[-3.14 +3.14]
This is particularly useful when you have large arrangements. The problem is that if you want to do it today, it is not implemented:
>>> print(f"{a:+.2f}")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to numpy.ndarray.__format__

In this PR ( I propose a very basic formatting implementation for numeric numbers that uses `array2string` just like it currently does `str`

At first, since we are only considering formatting the numeric type, floating numbers specifically, we are only interested in being able to change the precision, the sign, and possibly the rounding or truncation. Since the `array2string` function already does everything we need, we only need to implement the` __format__` function of the `ndarray` class which parses a predefined format (similar to the one already used by Python for built-in data types) to indicate the parameters before said.

I propose a mini format specification inspired in the [Format Specification Mini-Language](

format_spec ::=  [sign][.precision][type]
sign              ::=  "+" | "-" | " "
precision      ::=  [0-9]+
type             ::=  "f" | "e"

We are going to consider only 3 arguments of the `array2string` function:` precision`, `suppress_small`,` sign`. In particular, the `type` token sets the` suppress_small` argument to True when the type is `f` and False when it is `e`. This is in order to mimic Python's behavior in truncating decimals when using the fixed-point notation.

As @brandon-rhodes said in gh-5543, the behavior when you try to format an array containing Python objects, the behavior should be the same as Python has implemented by default in the `object` class: ` format (a, "") ` should be equivalent to `str (a)` and `format(a, "not empty")` should raise an exception.

What remains to be defined is the behavior when trying to format an array with a non-numeric data type (`np.numeric`) other than `np.object_`. Should we raise an exception? In my opinion yes, since in the future formatting is extended -- for example, for dates -- people are aware that before that was not implemented.

I'm open to suggestions.

- Ivan