[Bug]: invalid formatting of float/complex zero with missing format type

[skirpichev]

Describe the bug

Examples:

Python 3.12.8 (Tue Jun 23 15:26:01 UTC 2026)
[Graal, Oracle GraalVM, Java 25.0.3 (amd64)] on 'linux'
Type "help", "copyright", "credits" or "license" for more information.
>>> format(0j, '#.0')
'0j'
>>> format(0.0, '.0')
'0.0e+00'

vs

Python 3.12.13 (tags/v3.12.13:3bb231a6a5d, May 14 2026, 11:06:30) [GCC 14.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> format(0j, '#.0')
'0.j'
>>> format(0.0, '.0')
'0e+00'

Per documentation:

For float this is like the 'g' type, except that when fixed-point notation is used to format the result, it always includes at least one digit past the decimal point, and switches to the scientific notation when exp >= p - 1. When the precision is not specified, the latter will be as large as needed to represent the given value faithfully.

And for complex:

The available presentation types for complex are the same as those for float ('%' is not allowed). Both the real and imaginary components of a complex number are formatted as floating-point numbers, according to the specified presentation type. They are separated by the mandatory sign of the imaginary part, the latter being terminated by a j suffix. If the presentation type is missing, the result will match the output of str() (complex numbers with a non-zero real part are also surrounded by parentheses), possibly altered by other format modifiers.

Operating system

Linux

CPU architecture

x86_64

GraalPy version

25.1.3

JDK version

No response

Context configuration

No response

Steps to reproduce

See description

Expected behavior

Match CPython

Stack trace

Additional context

No response

[saisharan0103]

The zero cases here are narrowed to missing presentation type: format(0.0, '.0') keeps a decimal before the exponent, while format(0j, '#.0') drops the decimal before j.

For a PR, I would start by inspecting the float formatting path in FloatBuiltins.java where omitted type falls back to g-like behavior, then align the zero precision and alternate-form branches with CPython. Regression coverage should assert the exact float and complex strings shown here.