custom formatters: pass unit modifiers to the formatter

CHANGES

@@ -44,6 +44,7 @@ Pint Changelog
 - Fix setting options of the application registry (Issue #1403).
 - Fix Quantity & Unit `is_compatible_with` with registry active contexts (Issue #1424).
 - Allow Quantity to parse 'NaN' and 'inf(inity)', case insensitive
+- Pass unit modifiers to custom formatters (Issue #1448)
 - Fix casting error when using to_reduced_units with array of int.
   (Issue #1184)
 - Use default numpy `np.printoptions` available since numpy 1.15.

pint/facets/formatting/objects.py

@@ -22,7 +22,6 @@
     split_format,
 )
 from ...util import iterable
-from ..plain import UnitsContainer
 
 
 class FormattingQuantity:
@@ -186,42 +185,17 @@ def __format__(self, spec) -> str:
         _, uspec = split_format(
             spec, self.default_format, self._REGISTRY.separate_format_defaults
         )
-        if "~" in uspec:
-            if not self._units:
-                return ""
-            units = UnitsContainer(
-                dict(
-                    (self._REGISTRY._get_symbol(key), value)
-                    for key, value in self._units.items()
-                )
-            )
-            uspec = uspec.replace("~", "")
-        else:
-            units = self._units
 
-        return format_unit(units, uspec, registry=self._REGISTRY)
+        return format_unit(self, uspec, registry=self._REGISTRY)
 
     def format_babel(self, spec="", locale=None, **kwspec: Any) -> str:
         spec = spec or extract_custom_flags(self.default_format)
 
-        if "~" in spec:
-            if self.dimensionless:
-                return ""
-            units = UnitsContainer(
-                dict(
-                    (self._REGISTRY._get_symbol(key), value)
-                    for key, value in self._units.items()
-                )
-            )
-            spec = spec.replace("~", "")
-        else:
-            units = self._units
-
         locale = self._REGISTRY.fmt_locale if locale is None else locale
 
         if locale is None:
             raise ValueError("Provide a `locale` value to localize translation.")
         else:
             kwspec["locale"] = babel_parse(locale)
 
-        return units.format_babel(spec, registry=self._REGISTRY, **kwspec)
+        return self._units.format_babel(spec, registry=self._REGISTRY, **kwspec)

pint/formatting.py

@@ -163,10 +163,44 @@ def wrapper(func):
     return wrapper
 
 
+def apply_unit_modifiers(unit, modifiers, registry):
+    """apply modifiers to a unit
+
+    Parameters
+    ----------
+    unit : Unit or UnitsContainer
+        The input unit.
+    modifiers : str
+        The modifiers to apply.
+    registry : UnitRegistry
+        The unit registry associated with `unit`.
+
+    Returns
+    -------
+    UnitsContainer
+        A dict-like container with the preprocessed unit components.
+    """
+    from .util import UnitsContainer
+
+    raw = unit if isinstance(unit, UnitsContainer) else unit._units
+    if not raw:
+        return UnitsContainer({})
+
+    if "~" in modifiers:
+        applied = UnitsContainer(
+            {registry._get_symbol(key): value for key, value in raw.items()}
+        )
+    else:
+        applied = raw
+
+    return applied
+
+
 @register_unit_format("P")
-def format_pretty(unit, registry, **options):
+def format_pretty(unit, registry, modifiers, **options):
+    modified = apply_unit_modifiers(unit, modifiers, registry)
     return formatter(
-        unit.items(),
+        modified.items(),
         as_ratio=True,
         single_denominator=False,
         product_fmt="·",
@@ -179,9 +213,10 @@ def format_pretty(unit, registry, **options):
 
 
 @register_unit_format("L")
-def format_latex(unit, registry, **options):
+def format_latex(unit, registry, modifiers, **options):
+    modified = apply_unit_modifiers(unit, modifiers, registry)
     preprocessed = {
-        r"\mathrm{{{}}}".format(u.replace("_", r"\_")): p for u, p in unit.items()
+        r"\mathrm{{{}}}".format(u.replace("_", r"\_")): p for u, p in modified.items()
     }
     formatted = formatter(
         preprocessed.items(),
@@ -197,7 +232,7 @@ def format_latex(unit, registry, **options):
 
 
 @register_unit_format("Lx")
-def format_latex_siunitx(unit, registry, **options):
+def format_latex_siunitx(unit, registry, modifiers, **options):
     if registry is None:
         raise ValueError(
             "Can't format as siunitx without a registry."
@@ -206,14 +241,16 @@ def format_latex_siunitx(unit, registry, **options):
             " and might indicate a bug in `pint`."
         )
 
-    formatted = siunitx_format_unit(unit, registry)
+    modified = apply_unit_modifiers(unit, modifiers, registry)
+    formatted = siunitx_format_unit(modified, registry)
     return rf"\si[]{{{formatted}}}"
 
 
 @register_unit_format("H")
-def format_html(unit, registry, **options):
+def format_html(unit, registry, modifiers, **options):
+    modified = apply_unit_modifiers(unit, modifiers, registry)
     return formatter(
-        unit.items(),
+        modified.items(),
         as_ratio=True,
         single_denominator=True,
         product_fmt=r" ",
@@ -225,9 +262,10 @@ def format_html(unit, registry, **options):
 
 
 @register_unit_format("D")
-def format_default(unit, registry, **options):
+def format_default(unit, registry, modifiers, **options):
+    modified = apply_unit_modifiers(unit, modifiers, registry)
     return formatter(
-        unit.items(),
+        modified.items(),
         as_ratio=True,
         single_denominator=False,
         product_fmt=" * ",
@@ -239,9 +277,10 @@ def format_default(unit, registry, **options):
 
 
 @register_unit_format("C")
-def format_compact(unit, registry, **options):
+def format_compact(unit, registry, modifiers, **options):
+    modified = apply_unit_modifiers(unit, modifiers, registry)
     return formatter(
-        unit.items(),
+        modified.items(),
         as_ratio=True,
         single_denominator=False,
         product_fmt="*",  # TODO: Should this just be ''?
@@ -374,14 +413,15 @@ def formatter(
 # http://docs.python.org/2/library/string.html#format-specification-mini-language
 # We also add uS for uncertainties.
 _BASIC_TYPES = frozenset("bcdeEfFgGnosxX%uS")
+_UNIT_MODIFIERS = frozenset("~")
 
 
 def _parse_spec(spec):
     result = ""
     for ch in reversed(spec):
         if ch == "~" or ch in _BASIC_TYPES:
             continue
-        elif ch in list(_FORMATTERS.keys()) + ["~"]:
+        elif ch in list(_FORMATTERS.keys()) + list(_UNIT_MODIFIERS):
             if result:
                 raise ValueError("expected ':' after format specifier")
             else:
@@ -403,14 +443,15 @@ def format_unit(unit, spec, registry=None, **options):
         else:
             return "dimensionless"
 
-    if not spec:
-        spec = "D"
+    uspec, modifiers = split_unit_format(spec)
+    if not uspec:
+        uspec = "D"
 
-    fmt = _FORMATTERS.get(spec)
+    fmt = _FORMATTERS.get(uspec)
     if fmt is None:
         raise ValueError(f"Unknown conversion specified: {spec}")
 
-    return fmt(unit, registry=registry, **options)
+    return fmt(unit, registry=registry, modifiers=modifiers, **options)
 
 
 def siunitx_format_unit(units, registry):
@@ -455,26 +496,27 @@ def _tothe(power):
     return "".join(lpos) + "".join(lneg)
 
 
-def extract_custom_flags(spec):
-    import re
+def split_unit_format(uspec):
+    modifiers_re = re.compile(rf"[{''.join(_UNIT_MODIFIERS)}]")
+    modifiers = "".join(modifiers_re.findall(uspec))
+    uspec = modifiers_re.sub("", uspec)
 
-    if not spec:
-        return ""
+    return uspec, modifiers
 
-    # sort by length, with longer items first
-    known_flags = sorted(_FORMATTERS.keys(), key=len, reverse=True)
 
-    flag_re = re.compile("(" + "|".join(known_flags + ["~"]) + ")")
+def extract_custom_flags(spec):
+    flags = sorted(_FORMATTERS, key=len, reverse=True) + list(_UNIT_MODIFIERS)
+    flag_re = re.compile("|".join(flags))
+
     custom_flags = flag_re.findall(spec)
 
     return "".join(custom_flags)
 
 
 def remove_custom_flags(spec):
-    for flag in sorted(_FORMATTERS.keys(), key=len, reverse=True) + ["~"]:
-        if flag:
-            spec = spec.replace(flag, "")
-    return spec
+    flags = sorted(_FORMATTERS, key=len, reverse=True) + list(_UNIT_MODIFIERS)
+    flag_re = re.compile("|".join(flags))
+    return flag_re.sub("", spec)
 
 
 def split_format(spec, default, separate_format_defaults=True):

pint/testsuite/test_unit.py

@@ -124,6 +124,21 @@ def format_new(unit, **options):
 
         assert "{:new}".format(ureg.m) == "new format"
 
+    def test_unit_formatting_custom_modifiers(self, monkeypatch):
+        from pint import formatting, register_unit_format
+
+        monkeypatch.setattr(formatting, "_FORMATTERS", formatting._FORMATTERS.copy())
+
+        @register_unit_format("new")
+        def format_new(unit, *, modifiers, **options):
+            return f"new format for {unit:~P} with '{modifiers}'"
+
+        ureg = UnitRegistry()
+        u = ureg.m
+
+        assert f"{u:new}" == "new format for m with ''"
+        assert f"{u:~new}" == "new format for m with '~'"
+
     def test_ipython(self):
         alltext = []