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# Defining Quantities

A quantity in Pint is the product of a unit and a magnitude.

Pint supports several different ways of defining physical quantities, including a powerful string parsing system. These methods are largely interchangeable, though you may need to use the constructor form under certain circumstances (see Temperature conversion for an example of where the constructor form is required).

## By multiplication

If you’ve read the Tutorial, you’re already familiar with defining a quantity by multiplying a `Unit()` and a scalar:

    >>> from pint import UnitRegistry
    >>> ureg = UnitRegistry()
    >>> ureg.meter
    <Unit('meter')>
    >>> 30.0 * ureg.meter
    <Quantity(30.0, 'meter')>

This works to build up complex units as well:

    >>> 9.8 * ureg.meter / ureg.second**2
    <Quantity(9.8, 'meter / second ** 2')>

## Using the constructor

In some cases it is useful to define `Quantity()` objects using it’s class constructor. Using the constructor allows you to specify the units and magnitude separately.

We typically abbreviate that constructor as Q_ to make it’s usage less verbose:

    >>> Q_ = ureg.Quantity
    >>> Q_(1.78, ureg.meter)
    <Quantity(1.78, 'meter')>

As you can see below, the multiplication and constructor methods should produce the same results:

    >>> Q_(30.0, ureg.meter) == 30.0 * ureg.meter
    True
    >>> Q_(9.8, ureg.meter / ureg.second**2)
    <Quantity(9.8, 'meter / second ** 2')>

Quantity can be created with itself, if units is specified `pint` will try to convert it to the desired units. If not, pint will just copy the quantity.

    >>> length = Q_(30.0, ureg.meter)
    >>> Q_(length, 'cm')
    <Quantity(3000.0, 'centimeter')>
    >>> Q_(length)
    <Quantity(30.0, 'meter')>

## Using string parsing

Pint includes a powerful parser for detecting magnitudes and units (with or without prefixes) in strings. Calling the `UnitRegistry()` directly invokes the parsing function `UnitRegistry.parse_expression`:

    >>> 30.0 * ureg('meter')
    <Quantity(30.0, 'meter')>
    >>> ureg('30.0 meters')
    <Quantity(30.0, 'meter')>
    >>> ureg('3000cm').to('meters')
    <Quantity(30.0, 'meter')>

The parsing function is also available to the `Quantity()` constructor and the various `.to()` methods:

    >>> Q_('30.0 meters')
    <Quantity(30.0, 'meter')>
    >>> Q_(30.0, 'meter')
    <Quantity(30.0, 'meter')>
    >>> Q_('3000.0cm').to('meter')
    <Quantity(30.0, 'meter')>

Or as a standalone method on the `UnitRegistry`:

    >>> 2.54 * ureg.parse_expression('centimeter')
    <Quantity(2.54, 'centimeter')>

It is fairly good at detecting compound units:

    >>> g = ureg('9.8 meters/second**2')
    >>> g
    <Quantity(9.8, 'meter / second ** 2')>
    >>> g.to('furlongs/fortnight**2')
    <Quantity(7.12770743e+10, 'furlong / fortnight ** 2')>

And behaves well when given dimensionless quantities, which are parsed into dimensionless `Quantity` objects:

    >>> ureg('2.54')
    <Quantity(2.54, 'dimensionless')>
    >>> type(ureg('2.54'))
    <class 'pint.Quantity'>
    >>> Q_('2.54')
    <Quantity(2.54, 'dimensionless')>
    >>> type(Q_('2.54'))
    <class 'pint.Quantity'>

Note

Pint’s rule for parsing strings with a mixture of numbers and units is that units are treated with the same precedence as numbers.

For example, the units of

    >>> Q_('3 l / 100 km')
    <Quantity(0.03, 'liter * kilometer')>

may be unexpected at first but, are a consequence of applying this rule. Use brackets to get the expected result:

    >>> Q_('3 l / (100 km)')
    <Quantity(0.03, 'liter / kilometer')>

Special strings for NaN (Not a Number) and inf(inity) are also handled in a case-insensitive fashion. Note that, as usual, NaN != NaN.

    >>> Q_('inf m')
    <Quantity(inf, 'meter')>
    >>> Q_('-INFINITY m')
    <Quantity(-inf, 'meter')>
    >>> Q_('nan m')
    <Quantity(nan, 'meter')>
    >>> Q_('NaN m')
    <Quantity(nan, 'meter')>

Note

Since version 0.7, Pint does not use eval under the hood. This change removes the serious security problems that the system is exposed to when parsing information from untrusted sources.