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description: Placeholder expression.

Module: expression_impl.placeholder

View source on GitHub

Placeholder expression.

A placeholder expression represents prensor nodes, however a prensor is not needed until calculate is called. This allows the user to apply expression queries to a placeholder expression before having an actual prensor object. When calculate is called on a placeholder expression (or a descendant of a placeholder expression), the feed_dict will need to be passed in. Then calculate will bind the prensor with the appropriate placeholder expression.

Sample usage:

placeholder_exp = placeholder.create_expression_from_schema(schema)
new_exp = expression_queries(placeholder_exp, ..)
result = calculate.calculate_values([new_exp],
                                    feed_dict={placeholder_exp: pren})
# placeholder_exp requires a feed_dict to be passed in when calculating

Functions

create_expression_from_schema(...): Creates a placeholder expression from a parquet schema.

get_placeholder_paths_from_graph(...): Gets all placeholder paths from an expression graph.