# 7.11 Assertions

The `assert` command can be used to assert that a logical expression, such as `x>0`, is true. An error is reported if the expression is false, and optionally a string can be supplied to provide a more informative error message to the user:

    assert x>0
    assert y<0 "y must be less than zero."

The `assert` command can also be used to test the version number of Pyxplot. It is possible to test either that the version is newer than or equal to a specific version, using the `=` operator, or that it is older than a specific version, using the `` operator, as demonstrated in the following examples:

    assert version >= 0.8.2
    assert version <  0.8  "This script is designed for Pyxplot 0.7"