# InteractionTerm


A two-way parametric interaction between two bare covariates, e.g. `x1 * x2`.


Usage

``` python
InteractionTerm(
    left,
    right,
    full=True,
)
```


Represents a crossing of two columns in the model matrix (numeric-by-numeric products, numeric-by-factor varying slopes, or factor-by-factor cell means, depending on the column types). Use this when you want an interaction *without* smoothing -- for a smooth interaction surface between two continuous covariates, use a tensor-product [SmoothTerm](SmoothTerm.md#whittaker.SmoothTerm) (`te()`, `ti()`, or `t2()`) instead, or a factor-`by` [SmoothTerm](SmoothTerm.md#whittaker.SmoothTerm) for "one smooth per group".

`full=True` corresponds to `*`-style crossing (R's convention): both main effects (`x1` and `x2` individually) plus the interaction column(s) are included. `full=False` corresponds to `:`-style crossing: the interaction only, with no accompanying main effects. Note that the parser ([whittaker.formula.parser.parse](parse_formula.md#whittaker.parse_formula)) currently only produces terms with `full=True` (`x1 * x2` syntax); `x1:x2` colon syntax is not valid Python and is rejected at parse time, so `full=False` terms must be constructed directly if needed.


## Parameters


`left: str`  
Name of the first covariate.

`right: str`  
Name of the second covariate.

`full: bool = ``True`  
If `True` (default), include both main effects and the interaction (`*`-style). If `False`, include only the interaction, dropping the main effects (`:`-style).