154 lines
5.3 KiB
ReStructuredText
154 lines
5.3 KiB
ReStructuredText
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Pint: makes units easy
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======================
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.. image:: _static/logo-full.jpg
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:alt: Pint: **physical quantities**
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:class: floatingflask
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Pint is a Python package to define, operate and manipulate **physical quantities**:
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the product of a numerical value and a unit of measurement. It allows
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arithmetic operations between them and conversions from and to different units.
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It is distributed with a `comprehensive list of physical units, prefixes and constants`_.
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Due to its modular design, you can extend (or even rewrite!) the complete list
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without changing the source code. It supports a lot of numpy mathematical
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operations **without monkey patching or wrapping numpy**.
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It has a complete test coverage. It runs in Python 2.6+ and 3.2+ with no other
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dependency. It is licensed under BSD.
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It is extremely easy and natural to use:
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.. code-block:: python
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>>> import pint
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>>> ureg = pint.UnitRegistry()
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>>> 3 * ureg.meter + 4 * ureg.cm
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<Quantity(3.04, 'meter')>
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and you can make good use of numpy if you want:
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.. code-block:: python
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>>> import numpy as np
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>>> [3, 4] * ureg.meter + [4, 3] * ureg.cm
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<Quantity([ 3.04 4.03], 'meter')>
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>>> np.sum(_)
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<Quantity(7.07, 'meter')>
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Quick Installation
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------------------
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To install Pint, simply:
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.. code-block:: bash
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$ pip install pint
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or utilizing conda, with the conda-forge channel:
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.. code-block:: bash
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$ conda install -c conda-forge pint
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and then simply enjoy it!
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Design principles
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-----------------
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Although there are already a few very good Python packages to handle physical
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quantities, no one was really fitting my needs. Like most developers, I
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programmed Pint to scratch my own itches.
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**Unit parsing**: prefixed and pluralized forms of units are recognized without
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explicitly defining them. In other words: as the prefix *kilo* and the unit
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*meter* are defined, Pint understands *kilometers*. This results in a much
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shorter and maintainable unit definition list as compared to other packages.
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**Standalone unit definitions**: units definitions are loaded from a text file
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which is simple and easy to edit. Adding and changing units and their
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definitions does not involve changing the code.
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**Advanced string formatting**: a quantity can be formatted into string using
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`PEP 3101`_ syntax. Extended conversion flags are given to provide symbolic,
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LaTeX and pretty formatting. Unit name translation is available if Babel_ is
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installed.
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**Free to choose the numerical type**: You can use any numerical type
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(`fraction`, `float`, `decimal`, `numpy.ndarray`, etc). NumPy_ is not required
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but supported.
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**NumPy integration**: When you choose to use a NumPy_ ndarray, its methods and
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ufuncs are supported including automatic conversion of units. For example
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`numpy.arccos(q)` will require a dimensionless `q` and the units of the output
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quantity will be radian.
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**Uncertainties integration**: transparently handles calculations with
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quantities with uncertainties (like 3.14±0.01) meter via the `uncertainties
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package`_.
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**Handle temperature**: conversion between units with different reference
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points, like positions on a map or absolute temperature scales.
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**Small codebase**: easy to maintain codebase with a flat hierarchy.
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**Dependency free**: it depends only on Python and its standard library.
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**Python 2 and 3**: a single codebase that runs unchanged in Python 2.7+ and
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Python 3.0+.
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User Guide
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----------
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.. toctree::
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:maxdepth: 1
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getting
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tutorial
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numpy
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nonmult
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wrapping
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serialization
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pitheorem
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contexts
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measurement
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defining
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performance
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systems
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More information
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----------------
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.. toctree::
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:maxdepth: 1
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contributing
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faq
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One last thing
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--------------
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.. epigraph::
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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.
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`Mars Climate Orbiter Mishap Investigation Phase I Report`
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`PDF <ftp://ftp.hq.nasa.gov/pub/pao/reports/1999/MCO_report.pdf>`_
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.. _`comprehensive list of physical units, prefixes and constants`: https://github.com/hgrecco/pint/blob/master/pint/default_en.txt
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.. _`uncertainties package`: https://pythonhosted.org/uncertainties/
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.. _`NumPy`: http://www.numpy.org/
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.. _`PEP 3101`: https://www.python.org/dev/peps/pep-3101/
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.. _`Babel`: http://babel.pocoo.org/
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