# Pint: makes units easy¶

Pint is a Python package to define, operate and manipulate **physical quantities**:
the product of a numerical value and a unit of measurement. It allows
arithmetic operations between them and conversions from and to different units.

It is distributed with a comprehensive list of physical units, prefixes and constants.
Due to its modular design, you can extend (or even rewrite!) the complete list
without changing the source code. It supports a lot of numpy mathematical
operations **without monkey patching or wrapping numpy**.

It has a complete test coverage. It runs in Python 2.6+ and 3.2+ with no other dependency. It is licensed under BSD.

It is extremely easy and natural to use:

```
>>> import pint
>>> ureg = pint.UnitRegistry()
>>> 3 * ureg.meter + 4 * ureg.cm
<Quantity(3.04, 'meter')>
```

and you can make good use of numpy if you want:

```
>>> import numpy as np
>>> [3, 4] * ureg.meter + [4, 3] * ureg.cm
<Quantity([ 3.04 4.03], 'meter')>
>>> np.sum(_)
<Quantity(7.07, 'meter')>
```

## Design principles¶

Although there are already a few very good Python packages to handle physical quantities, no one was really fitting my needs. Like most developers, I programmed Pint to scratch my own itches.

**Unit parsing**: prefixed and pluralized forms of units are recognized without
explicitly defining them. In other words: as the prefix *kilo* and the unit
*meter* are defined, Pint understands *kilometers*. This results in a much
shorter and maintainable unit definition list as compared to other packages.

**Standalone unit definitions**: units definitions are loaded from a text file
which is simple and easy to edit. Adding and changing units and their
definitions does not involve changing the code.

**Advanced string formatting**: a quantity can be formatted into string using
PEP 3101 syntax. Extended conversion flags are given to provide symbolic,
LaTeX and pretty formatting.

**Free to choose the numerical type**: You can use any numerical type
(fraction, float, decimal, numpy.ndarray, etc). NumPy is not required
but supported.

**NumPy integration**: When you choose to use a NumPy ndarray, its methods and
ufuncs are supported including automatic conversion of units. For example
numpy.arccos(q) will require a dimensionless q and the units of the output
quantity will be radian.

**Uncertainties integration**: transparently handles calculations with
quantities with uncertainties (like 3.14±0.01) meter via the uncertainties
package.

**Handle temperature**: conversion between units with different reference
points, like positions on a map or absolute temperature scales.

**Small codebase**: easy to maintain codebase with a flat hierarchy.

**Dependency free**: it depends only on Python and its standard library.

**Python 2 and 3**: a single codebase that runs unchanged in Python 2.7+ and
Python 3.0+.

## User Guide¶

## More information¶

## One last thing¶

The MCO MIB has determined that the root cause for the loss of the MCO spacecraft was the failure to use metric units in the coding of a ground software file, “Small Forces,” used in trajectory models. Specifically, thruster performance data in English units instead of metric units was used in the software application code titled SM_FORCES (small forces). The output from the SM_FORCES application code as required by a MSOP Project Software Interface Specification (SIS) was to be in metric units of Newtonseconds (N-s). Instead, the data was reported in English units of pound-seconds (lbf-s). The Angular Momentum Desaturation (AMD) file contained the output data from the SM_FORCES software. The SIS, which was not followed, defines both the format and units of the AMD file generated by ground-based computers. Subsequent processing of the data from AMD file by the navigation software algorithm therefore, underestimated the effect on the spacecraft trajectory by a factor of 4.45, which is the required conversion factor from force in pounds to Newtons. An erroneous trajectory was computed using this incorrect data.

Mars Climate Orbiter Mishap Investigation Phase I Report PDF