Generalized indexing, correct formatting for arrays, new constants.

CHANGELOG.md

@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- Electric and Coulomb constants added. [#47](https://github.com/itt-ustutt/quantity/issues/47)
+- ATM and POISE added as derived units. [#46](https://github.com/itt-ustutt/quantity/issues/46)
 
 ## [0.13.0] - 2026-01-06
 ### Changed

si-units/CHANGELOG.md

@@ -5,6 +5,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Fixed 
+- Correctly formats output of arrays with a unit. 
+
+### Added
+- Getter and Setter generalized to common NumPy indexing, such that general slicing operations are possible in multiple dimensions. 
 
 ## [0.11.1] - 2026-01-22
 ### Fixed

si-units/docs/derived.md

@@ -23,28 +23,32 @@ For convenience, a number of commonly used units that are not directly combinati
 These constants simplify the specification of properties, that are not given in SI units. However, as the representation
 of quantities is unique, they do not appear in formatted outputs.
 
-| Unit     | Unit symbol  | Quantity      | Definition                                    |
-| -------- | ------------ | ------------- | --------------------------------------------- |
-| ANGSTROM | $\text{Å}$   | length        | $10^{-10}~\text{m}$                           |
-| AMU      | $\text{u}$   | mass          | $1.6605390671738466\times 10^{-27}~\text{kg}$ |
-| AU       | $\text{au}$  | length        | $149597870700~\text{m}$                       |
-| BAR      | $\text{bar}$ | pressure      | $10^5~\text{Pa}$                              |
-| CALORIE  | $\text{cal}$ | energy        | $4.184~\text{J}$                              |
-| DAY      | $\text{d}$   | time          | $86400~\text{s}$                              |
-| DEBYE    | $\text{De}$  | dipole moment | $\sqrt{10^{-19}~\text{JÅ}^3}$                 |
-| DEGREES  | $^\circ$     | angle         | $\frac{\pi}{180}~\text{rad}$                  |
-| GRAM     | $\text{g}$   | mass          | $10^{-3}~\text{kg}$                           |
-| HOUR     | $\text{h}$   | time          | $3600~\text{s}$                               |
-| LITER    | $\text{l}$   | volume        | $10^{-3}~\text{m}^3$                          |
-| MINUTE   | $\text{min}$ | time          | $60~\text{s}$                                 |
-| RADIANS  | $\text{rad}$ | angle         |                                               |
+| Unit     | Unit symbol     | Quantity      | Definition                                    |
+| -------- | --------------- | ------------- | --------------------------------------------- |
+| ANGSTROM | $\text{Å}$      | length        | $10^{-10}~\text{m}$                           |
+| AMU      | $\text{u}$      | mass          | $1.6605390671738466\times 10^{-27}~\text{kg}$ |
+| ATM      | $\text{bar}$    | presssure     | $101325~\text{N/m^2}$                         |
+| AU       | $\text{au}$     | length        | $149597870700~\text{m}$                       |
+| BAR      | $\text{bar}$    | pressure      | $10^5~\text{Pa}$                              |
+| CALORIE  | $\text{cal}$    | energy        | $4.184~\text{J}$                              |
+| DAY      | $\text{d}$      | time          | $86400~\text{s}$                              |
+| DEBYE    | $\text{De}$     | dipole moment | $\sqrt{10^{-19}~\text{JÅ}^3}$                 |
+| DEGREES  | $^\circ$        | angle         | $\frac{\pi}{180}~\text{rad}$                  |
+| GRAM     | $\text{g}$      | mass          | $10^{-3}~\text{kg}$                           |
+| HOUR     | $\text{h}$      | time          | $3600~\text{s}$                               |
+| LITER    | $\text{l}$      | volume        | $10^{-3}~\text{m}^3$                          |
+| MINUTE   | $\text{min}$    | time          | $60~\text{s}$                                 |
+| POISE    | $\text{m Pa s}$ | viscosity     | $0.1~\text{Pa s}$                             |
+| RADIANS  | $\text{rad}$    | angle         |                                               |
 
 ## Additional Constants
 
-| Constant | Name                   | Symbol                    | Value                                                               |
-| -------- | ---------------------- | ------------------------- | ------------------------------------------------------------------- |
-| G        | Gravitational constant | $G$                       | $6.6743\times 10^{-11}~\frac{\text{m}^3}{\text{kg}\cdot\text{s}^2}$ |
-| RGAS     | Ideal gas constant     | $R=N_\text{Av}k_\text{B}$ | $8.31446261815324~\frac{\text{J}}{\text{mol}\cdot\text{K}}$         |
+| Constant | Name                   | Symbol                                    | Value                                                                  |
+| -------- | ---------------------- | ----------------------------------------- | ---------------------------------------------------------------------- |
+| G        | Gravitational constant | $G$                                       | $6.6743\times 10^{-11}~\frac{\text{m}^3}{\text{kg}\cdot\text{s}^2}$    |
+| EPSILON0 | Electric constant      | $\epsilon_0$                              | $8.8541878188e-12~\frac{\text{A}\cdot\text{s}}{\text{V}\cdot\text{m}}$ |
+| KE       | Coulomb constant       | $k_\text{e} = \frac{1}{4 \pi \epsilon_0}$ | $8987551786.170797~\frac{\text{N}\cdot\text{m}^2}{\text{C}^2}$         |
+| RGAS     | Ideal gas constant     | $R=N_\text{Av}k_\text{B}$                 | $8.31446261815324~\frac{\text{J}}{\text{mol}\cdot\text{K}}$            |
 
 ### Example
 

si-units/src/fmt.rs

@@ -73,6 +73,15 @@ impl SINumber {
             format!("{}\\,{}", float_to_latex(self.value), &self.unit.to_latex())
         }
     }
+
+    pub fn into_scaled_parts(&self) -> (f64, String) {
+        if let Some((unit, symbol, _, _, _)) = DERIVED_UNITS.get(&self.unit) {
+            if !self.value.is_nan() {
+                return (self.value / unit.value, symbol.clone());
+            }
+        }
+        (self.value, self.unit.to_string())
+    }
 }
 
 fn float_to_latex(f: f64) -> String {

si-units/src/lib.rs

@@ -1,5 +1,7 @@
 #![warn(clippy::all)]
 #![allow(non_snake_case)]
+use std::f64::consts::PI;
+
 use ndarray::{Array1, arr1};
 use numpy::IntoPyArray;
 use pyo3::basic::CompareOp;
@@ -78,11 +80,20 @@ impl PySIObject {
         if let Ok(v) = self.value.extract::<f64>(py) {
             Ok(SINumber::new(v, self.unit).to_string())
         } else {
-            let value = self
-                .value
-                .call_method0(py, "__repr__")?
-                .extract::<String>(py)?;
-            Ok(format!("{} {}", value, self.unit))
+            let (multiplier, symbol) = SINumber::new(1.0, self.unit).into_scaled_parts();
+
+            let value_str = if (multiplier - 1.0).abs() > 1e-15 {
+                let scaled_val = self.value.call_method1(py, "__mul__", (multiplier,))?;
+                scaled_val
+                    .call_method0(py, "__repr__")?
+                    .extract::<String>(py)?
+            } else {
+                self.value
+                    .call_method0(py, "__repr__")?
+                    .extract::<String>(py)?
+            };
+
+            Ok(format!("{} {}", value_str, symbol))
         }
     }
 
@@ -258,25 +269,43 @@ impl PySIObject {
             .and_then(|v| v.extract::<usize>(py))
     }
 
-    fn __getitem__(&self, py: Python, idx: isize) -> PyResult<Self> {
+    fn __getitem__(&self, py: Python, idx: &Bound<'_, PyAny>) -> PyResult<Self> {
         let value = self.value.call_method1(py, "__getitem__", (idx,))?;
         Ok(Self::new(value, self.unit))
     }
 
-    fn __setitem__(&self, py: Python, idx: isize, value: SINumber) -> PyResult<()> {
-        if self.unit == value.unit {
+    fn __setitem__(
+        &self,
+        py: Python,
+        idx: &Bound<'_, PyAny>,
+        value: &Bound<'_, PyAny>,
+    ) -> PyResult<()> {
+        if let Ok(v) = value.extract::<SINumber>() {
+            if self.unit == v.unit {
+                self.value.call_method1(py, "__setitem__", (idx, v.value))?;
+                return Ok(());
+            }
+            return Err(QuantityError::InconsistentUnits {
+                unit1: self.unit,
+                unit2: v.unit,
+            }
+            .into());
+        }
+
+        let ob = value.extract::<PyRef<PySIObject>>()?;
+        if self.unit == ob.unit {
             self.value
-                .call_method1(py, "__setitem__", (idx, value.value))?;
+                .call_method1(py, "__setitem__", (idx, &ob.value))?;
             Ok(())
         } else {
             Err(QuantityError::InconsistentUnits {
                 unit1: self.unit,
-                unit2: value.unit,
-            })?
+                unit2: ob.unit,
+            }
+            .into())
         }
     }
 }
-
 #[derive(Clone, Copy)]
 pub struct SIObject<T> {
     value: T,
@@ -396,6 +425,9 @@ const _TESLA: SIUnit = SIUnit([0, 1, -2, -1, 0, 0, 0]);
 const _HENRY: SIUnit = SIUnit([2, 1, -2, -2, 0, 0, 0]);
 const _METER_PER_SECOND: SIUnit = SIUnit([1, 0, -1, 0, 0, 0, 0]);
 const _LUMEN_PER_WATT: SIUnit = SIUnit([-2, -1, 3, 0, 0, 0, 1]);
+const _FARAD_PER_METER: SIUnit = SIUnit([-3, -1, 4, 2, 0, 0, 0]);
+const _METER_PER_FARAD: SIUnit = SIUnit([3, 1, -4, -2, 0, 0, 0]);
+const _PASCAL_SECOND: SIUnit = SIUnit([-1, 1, -1, 0, 0, 0, 0]);
 
 /// Prefix quecto $\\left(\text{q}=10^{-30}\\right)$
 pub const QUECTO: f64 = 1e-30;
@@ -471,6 +503,13 @@ pub fn _core(m: &Bound<'_, PyModule>) -> PyResult<()> {
     add_constant(m, "KB", 1.380649e-23, _JOULE_PER_KELVIN)?;
     add_constant(m, "NAV", 6.02214076e23, _PER_MOL)?;
     add_constant(m, "KCD", 683.0, _LUMEN_PER_WATT)?;
+    add_constant(m, "EPSILON0", 8.8541878188e-12, _FARAD_PER_METER)?;
+    add_constant(
+        m,
+        "KE",
+        1.0_f64 / (4.0_f64 * PI * 8.8541878188e-12),
+        _METER_PER_FARAD,
+    )?;
 
     add_constant(m, "HERTZ", 1.0, _HERTZ)?;
     add_constant(m, "NEWTON", 1.0, _NEWTON)?;
@@ -489,6 +528,7 @@ pub fn _core(m: &Bound<'_, PyModule>) -> PyResult<()> {
     add_constant(m, "ANGSTROM", 1e-10, _METER)?;
     add_constant(m, "AMU", 1.6605390671738466e-27, _KILOGRAM)?;
     add_constant(m, "AU", 149597870700.0, _METER)?;
+    add_constant(m, "ATM", 101325.0, _PASCAL)?;
     add_constant(m, "BAR", 1e5, _PASCAL)?;
     add_constant(m, "CALORIE", 4.184, _JOULE)?;
     m.add("CELSIUS", Celsius)?;
@@ -499,6 +539,7 @@ pub fn _core(m: &Bound<'_, PyModule>) -> PyResult<()> {
     add_constant(m, "HOUR", 3600.0, _SECOND)?;
     add_constant(m, "LITER", 1e-3, _CUBIC_METER)?;
     add_constant(m, "MINUTE", 60.0, _SECOND)?;
+    add_constant(m, "POISE", 0.1, _PASCAL_SECOND)?;
     m.add("RADIANS", Angle(1.0))?;
 
     add_constant(m, "G", 6.6743e-11, SIUnit([3, -1, -2, 0, 0, 0, 0]))?;

si-units/src/si_units/__init__.py

@@ -17,6 +17,7 @@
     >>> print(f"The pressure is {pressure / BAR:.2f} bar")
     The pressure is 1.24 bar
 """
+
 # from si_units._core import SIObject
 from si_units._core import (
     SIObject,
@@ -43,6 +44,7 @@
     KB,
     NAV,
     KCD,
+    KE,
     HERTZ,
     NEWTON,
     PASCAL,
@@ -58,13 +60,16 @@
     HENRY,
     ANGSTROM,
     AMU,
+    ATM,
     AU,
     BAR,
     CALORIE,
+    EPSILON0,
     GRAM,
     HOUR,
     LITER,
     MINUTE,
+    POISE,
     G,
     RGAS,
     QUECTO,
@@ -119,6 +124,7 @@
     "KB",
     "NAV",
     "KCD",
+    "KE",
     "HERTZ",
     "NEWTON",
     "PASCAL",
@@ -134,13 +140,16 @@
     "HENRY",
     "ANGSTROM",
     "AMU",
+    "ATM",
     "AU",
     "BAR",
     "CALORIE",
+    "EPSILON0",
     "GRAM",
     "HOUR",
     "LITER",
     "MINUTE",
+    "POISE",
     "G",
     "RGAS",
     "QUECTO",

src/fmt.rs

@@ -44,6 +44,8 @@ impl<Inner: fmt::Display> fmt::Display for Quantity<Inner, _Dimensionless> {
 #[cfg(feature = "pyo3")]
 pub(crate) trait PrintUnit {
     const UNIT: &'static str;
+
+    fn scale() -> f64;
 }
 
 macro_rules! impl_fmt {
@@ -95,6 +97,10 @@ macro_rules! impl_fmt {
         #[cfg(feature = "python")]
         impl<T> PrintUnit for Quantity<T, SIUnit<$t, $l, $m, $i, $theta, $n, 0>> {
             const UNIT: &'static str = $symbol;
+
+            fn scale() -> f64 {
+                $unit.0
+            }
         }
     };
 }

src/lib.rs

@@ -531,6 +531,10 @@ pub const HOUR: Time = Quantity(3600.0, PhantomData);
 pub const LITER: Volume = Quantity(1e-3, PhantomData);
 /// Additional unit minute $\\left(1\\,\text{min}=60,\text{s}\\right)$
 pub const MINUTE: Time = Quantity(60.0, PhantomData);
+/// Additional unit atmosphere $\left(1\,\text{atm}=101325\,\text{Pa}\right)$
+pub const ATM: Pressure = Quantity(101325.0, PhantomData);
+/// Additional unit poise $\left(1\,\text{P}=0.1\,\text{Pa}\cdot\text{s}\right)$
+pub const POISE: Viscosity = Quantity(0.1, PhantomData);
 
 /// Boltzmann constant $\\left(k_\text{B}=1.380649\times 10^{-23}\\,\\frac{\text{J}}{\text{K}}\\right)$
 pub const KB: Entropy = Quantity(1.380649e-23, PhantomData);