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# Properties¶

Properties in Mantid are the mechanism by which we pass parameters into algorithms. There are a number of different types of properties, and these are described below.

## Types of Properties¶

### Single Value Properties¶

This is the simplest type of property, which is essentially a name-value pair. Currently, single value properties of type integer (int), floating point (double), string (std::string) and boolean (bool) are supported. .

### Array Properties¶

Sometimes, a multi-element parameter may be required (a list of spectra to process, for example). This is achieved using an ArrayProperty (which is actually a PropertyWithValue - see above - of type std::vector). It can be created in a number of ways:

• As an empty vector: ArrayProperty<double>("PropertyName");

• By passing in an existing vector: ArrayProperty<double>("PropertyName",myStdVecOfDoubles);

• By passing in a string of comma-separated values: ArrayProperty<int>("PropertyName","1,2,3,4");

A validator (see below) argument can optionally be appended to all of these options.

An ArrayProperty can be declared in a algorithm as follows:

declareProperty(std::make_unique<ArrayProperty>(...));

(note the use of std::make_unique rather than new, as the algorithm takes ownership of the property)

or, if creating using an already existing vector:

declareProperty("PropertyName",myVector);

### File Properties¶

There are two file properties: FileProperty and MultipleFileProperty. These properties are for capturing and holding the path and filenames to external file (s). File properties have a FileAction property that controls it’s purpose and behaviour.

Save :to specify a file to write to, the file may or may not exist OptionalSave :to specify a file to write to but an empty string is allowed here which will be passed to the algorithm Load :to specify a file to open for reading, the file must exist OptionalLoad :to specify a file to read but the file doesn’t have to exist

If the file property is has a FileAction of Load as is given a relative path (such as “input.txt” or “\data\input.txt” as its value it will search for matching files in this order:

Note, that MultipleFileProperty supports only Load (default) and OptionalLoad actions.

1. The current directory

2. The entries in order from the datasearch.directories entry in the Properties File

If the file property is has a FileAction of Save as is given a relative path (such as “input.txt” or “\data\input.txt” as its value it will assume that path starts from the location defined in the defaultsave.directory entry in the Properties File.

A FileProperty can be declared in a algorithm as follows:

declareProperty(new Kernel::FileProperty("Filename","",
  Kernel::FileProperty::Load), “Descriptive text”);

or for saving a file providing a suggested extension

declareProperty(new Kernel::FileProperty("Filename","",
  Kernel::FileProperty::Save, 
  std::vector(1,"cal")), "Descriptive text");

### Workspace Properties¶

Properties for holding workspaces are more complicated, in that they need to hold links both to the workspace name (in the Analysis Data Service) and the workspace itself. When setting or retrieving the value as a string (i.e. using the setValue or value methods) you are interacting with the workspace’s name; other methods interact with a shared pointer to the workspace.

The syntax to declare a WorkspaceProperty in an algorithm is:

declareProperty(std::make_unique<WorkspaceProperty<>>("PropertyName","WorkspaceName",direction));

In this case, the direction (see below) must be explicitly declared. An optional validator may also be appended to the above declaration.

Note that the algorithm takes ownership of the property, and that it is passed in as a unique_ptr rather than creating it with new.

## Other ‘Property Properties’¶

### Default values¶

If a property is empty

• in a GUI algorithm call, then the property’s default value is used (if there is any)

• in a Python API call, then the property’s value is left empty.

### Direction¶

All properties have a direction. They can be input or output properties, or both. The default is always input. Technically, these are a C++ enum, which can have the following values:

Mantid::Kernel::Direction::Input
Mantid::Kernel::Direction::Output
Mantid::Kernel::Direction::InOut

This is what should be passed in when a direction argument is required. The InOut option is principally used by workspace properties, when a single workspace is to be input and manipulated by as algorithm rather than a new one created to store the result.

### Validators¶

A validator is an external object that is used to verify that the value of a property is suitable for a particular algorithm. If no validator is given, then the property can have any value (of the correct type). Validators are checked immediately before an algorithm is executed, when the value of a property is set (which will fail if it doesn’t pass the validator) and through the GUI interface to an algorithm.

The validators currently included in Mantid are:

• BoundedValidator - restricts a numeric property to a particular range.

• MandatoryValidator - requires that a string or array property not be empty.

• ListValidator - restricts a string property to one of a particular set of values.

• FileValidator - ensures that a file (given as a string property) exists (used internally by the FileProperty).

• MultiFileValidator - ensures that each file in a given list exists (used internally by the MultipleFileProperty).

In addition, there are a number of validators specifically for use with Workspace properties:

• InstrumentValidator - checks that the workspace has an Instrument object.

• WorkspaceUnitValidator - checks that the workspace has a specified unit.

• HistogramValidator - requires that the workspace contains histogram data (or not).

• RawCountValidator - requires that the workspace data is raw counts.

• CommonBinsValidator - checks that all spectra in a workspace have the same bins.

• EqualBinSizesValidator - checks that all bins in each spectrum are equally sized, up to a given tolerance.

• SpectraAxisValidator - checks that the axis of the workspace contains spectra numbers.

• NumericAxisValidator - checks that the axis of the workspace contains numeric data.

• CompositeValidator - enables combination of more that one of the above validators for the same WorkspaceProperty.

In addition to the above, if used, Workspace properties also have a built in validator that requires that input workspaces exist and are of the correct type and that output workspaces have a name set.

For more details on using validators, see the PropertyAlgorithm example or the full documentation for the individual validators (linked above).

Writing your own validator is relatively straightforward - it simply has to implement the IValidator interface.

Category: Concepts