Class Param

source code

                   object --+                    
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 base.structure.StructureBase --+                
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                   object --+   |                
                            |   |                
           base.common.Parser --+                
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base.structure.ParseableStructure --+            
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             base.structure.Structure --+        
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                           object --+   |        
                                    |   |        
                  base.meta.MetaMixin --+        
                                        |        
                               ColumnBase --+    
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                                    ParamBase --+
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                                               Param

A table parameter.

This is like a column, except that it conceptually applies to all rows in the table. In VOTables, params will be rendered as PARAMs.

While we validate the values passed using the DC default parsers, at least the VOTable params will be literal copies of the string passed in.

You can obtain a parsed value from the value attribute.

Null value handling is a bit tricky with params. An empty param (like <param name="x"/>) is always NULL (None in python). In order to allow setting NULL even where syntactially something has to stand, we also turn any __NULL__ to None.

For floats, NaN will also yield NULLs. For integers, you can also use

<param name="x" type="integer"><values nullLiteral="-1"/>-1</params>

For arrays, floats, and strings, the interpretation of values is undefined. Following VOTable practice, we do not tell empty strings and NULLs apart; for internal usage, there is a little hack: __EMPTY__ as literal does set an empty string. This is to allow defaulting of empty strings -- in VOTables, these cannot be distinguished from "true" NULLs.