parse_expr

sherpa.utils.parse_expr(expr)

Convert a filter expression into its parts.

This is intended for parsing a notice or ignore expression given as a string.

Parameters

expr (str) – The filter expression, of the form ‘a:b’ or a single number, separated by commas, and white space is ignored. The upper or lower limit of a pair may be ignored (e.g. ‘a:’ or ‘:b’).

Returns

filters – Each pair gives the lower- and upper-edge of the filter, using None to represent no limit.

Return type

list of pairs

Notes

There is no attempt to validate that the expression contains strictly ordered pairs, or that the pairs do not overlap, or that the lower- and upper-limits are in increasing numerical order. That is, the expression ‘5:7,:2,4:6,5:3’ is allowed.

Examples

>>> parse_expr('0.5:7')
[(0.5, 7.0)]

>>> parse_expr('0.5:')
[(0.5, None)]

>>> parse_expr(':7')
[(None, 7.0)]

>>> parse_expr(':2, 4 : 5 ,7:8,10:')
[(None, 2.0), (4.0, 5.0), (7.0, 8.0), (10.0, None)]

>>> parse_expr('4')
[(4.0, 4.0)]

>>> parse_expr(' ')
[(None, None)]