## system.Parameter


Define a measurement parameter.


Usage

``` python
system.Parameter(
    name,
    unit,
    setter=None,
    getter=None,
    default=None,
    dtypes=None,
    chunks=None,
    trigger=None,
    setter_args=None,
    setter_kwargs=None,
    getter_args=None,
    getter_kwargs=None,
    trigger_args=None,
    trigger_kwargs=None,
    label=None
)
```


This class describes one parameter in matrix. It can define a single or multiple columns of the measurement.


## Parameters


`name: str or list of str`  
Name of the column(s) as string or list of strings. If this is a list, make sure unit, default and chunks have same length.

`unit: str or list of str`  
Unit of the column(s) as string or list of strings.

`default: float or list of floats = None`  
Default value for parameter. If not None this value is always used unless another value is specified in the measurement. If None (default), no default value is set/used.

`dtypes: str or list of str = None`  
Dtype specified for saving into hdf5 files, not used for ascii files. Default value is "f8" (8 byte float).

`chunks: int or list of int = None`  
Length of the readback value. If a list is returned for a single parameter, set to the length of that list. If None (default), a chunk of 1 is assumed (readback of parameter is single float).

`setter: callable, str, or list = None`  
Function which should be called to set the values. Must be one of:

- A callable function with the call signature `func(value, *args, **kwargs)`. For optional arguments and kwargs see setter_args/setter_kwargs.
- A string with a system method/property name. If it corresponds to a method its call signature and arguments must be equal to the callable function above.
- A list of the following scheme \[device_name : str, method_name : str, args : tuple, kwargs : dict\]. The args and kwargs entries are deprecated and should be replaced by the setter_args, setter_kwargs parameters.

`getter: callable, str, or list = None`  
Function which should be called to fetch the values. Must be one of:

- A callable function with the call signature `func(*args, **kwargs)`. The arguments and kwargs are optional and can be supplied via getter_args/getter_kwargs.
- A string with a system method/property name. If it corresponds to a method its call signature and arguments must be equal to the callable function above.
- A list of the following scheme \[device_name : str, method_name : str, args : tuple, kwargs : dict\]. The args and kwargs entries are deprecated and should be replaced by the getter_args, getter_kwargs parameters.

`trigger: callable, str, or list = None`  
Takes a trigger function. The options are equal to the getter options. For the optional arguments and kwargs use trigger_args/trigger_kwargs.

`label: str = None`  
Parameters label if different from name. This might be in particular needed if an automatically generated label from a name-list is not describing the content very well.


## Attributes


`All parameters are set as attributes of same name.`  


## Raises


`ValueError`  
If parameter types or lengths are inconsistent.

`TypeError`  
If parameter types are incorrect.