[Python-ideas] proposal to add rowexpr as a keyword

[Steve Howell]

--- Steven Bethard <steven.bethard at gmail.com> wrote:
> >>> def rowexpr(expr):
> ...     def evaluate(row):
> ...         d = dict(row)
> ...         d.update(globals())
> ...         exec '__result__ = ' + expr in d
> ...         return d['__result__']
> ...     return evaluate
> ...
> >>> def convert_to_euros(amt):
> ...     return amt / 2
> ...
> >>> x = rowexpr('convert_to_euros(salary)')
> >>> row = dict(salary=50000)
> >>> print x(row)
> 25000
> 

Thanks.

I tried it out with a slightly more involved
expression.

x = rowexpr(
        'convert_to_euros(salary) '
        'if dept == "foo" else 0')
rows = [dict(dept='foo', salary=i) for i in
range(10000)]
transform = [x(row) for row in rows]

Here were my findings:

1) Not horribly slow.

    3.4 seconds on my box for 10000 calls

2) I introduced a syntax error, and it was more clear
than I thought it would be.  It happens at runtime, of
course, which is less than ideal, and it doesn't
directly point me out to the line of code with the
syntax error, but it does suggest the error:

  File "sql.py", line 14, in <module>
    transform = [x(row) for row in rows]
  File "sql.py", line 5, in evaluate
    exec '__result__ = ' + expr in d
  File "<string>", line 1
    __result__ = convert_to_euros(salary) if dept =
"foo" else 0
                                                  ^
SyntaxError: invalid syntax





 
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