# 4.2 Physical constants

Many mathematical and physical constants are pre-defined in Pyxplot. A complete list of these can be found Chapter 3. Some of these, for example, `e`, `pi` and `goldenRatio` are standard mathematical constants which are accessible in the user’s default namespace:

`pyxplot> print pi`
`3.1415927`

Others, such as physical constants, are of more specialist interest and are defined in modules. For example, the speed of light is defined in the physics module `phy`:

`pyxplot> print phy.c`
`299792.46 km/s`

Most of the pre-defined physical constants, such as this one, make use of Pyxplot’s native ability to keep track of the physical units of quantities and to convert them between different unit systems – for example, between inches and centimeters. This will be explained in more detail in Section 4.6.

To list all of the functions and variables defined in a module such as `phy`, type

    print phy

or simply

    phy