Re: eval [was Re: dict to boolean expression, how to?]

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From	Peter Otten <__peter__@web.de>
Subject	Re: eval [was Re: dict to boolean expression, how to?]
Date	Sat, 02 Aug 2014 09:43:54 +0200
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Steven D'Aprano wrote:

> On Fri, 01 Aug 2014 17:44:27 +0200, Peter Otten wrote:
> [...]
>>> bool = ((df['a'] == 1) & (df['A'] == 0) |
>>>          (df['b'] == 1) & (df['B'] == 0) |
>>>          (df['c'] == 1) & (df['C'] == 0))
>>  
>> This is how it might look without eval():
>> 
>> #untested
>> result = functools.reduce(operator.or_, ((v == 1) & (df[k.upper()] == 0)
>> for k, v in df.items() if k.islower()))
> 
> For those who agree with Guido that reduce makes code unreadable:
> 
> result = True
> for key in df:
>     if key.islower():
>         result = result or (df[key] == 1 and df[key.upper()] == 0)

I cheated a bit and gave the solution that the OP was unlikely to come up 
with ;)


> Or if you insist on a single expression:
> 
> result = any(df[k] == 1 and df[k.upper()] == 0 for k in df if k.islower())
> 
> 
>> And here is an eval-based solution:
>> 
>> # untested
>> expr = "|".join(
>>     "((df[{}] == 1) | (df[{}] == 0))".format(c, c.upper()) for c in df
>>     is c.islower())
>> result = eval(expr)
> 
> I really don't believe that there is any benefit to that in readability,
> power, flexibility, or performance. 

You can put in a print(expr) to verify that it's identical to the original 
spelt out expression.

> Also, you're using bitwise operators
> instead of shortcut bool operators. Any reason why?

Since he started the thread Alex has moved the goal posts a bit and added 
that he is using a pandas DataFrame. That works much like a numpy array. So:

>>> df
   a  A    b     B   c   C
0  1  4  100  1000   7  14
1  2  1  200  2000  21  28
2  1  0  300  3000  35  42
>>> df["a"] == 1
0     True
1    False
2     True
Name: a, dtype: bool
>>> (df["a"] == 1) and (df["A"] == 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: The truth value of an array with more than one element is 
ambiguous. Use a.any() or a.all()
>>> (df["a"] == 1) & (df["A"] == 0)
0    False
1    False
2     True
dtype: bool

Thread

dict to boolean expression, how to? Alex van der Spek <zdoor@xs4all.nl> - 2014-08-01 12:45 +0000
  Re: dict to boolean expression, how to? Chris Angelico <rosuav@gmail.com> - 2014-08-01 23:04 +1000
  Re: dict to boolean expression, how to? Roy Smith <roy@panix.com> - 2014-08-01 09:24 -0400
  Re: dict to boolean expression, how to? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-01 13:28 +0000
    Re: dict to boolean expression, how to? Roy Smith <roy@panix.com> - 2014-08-01 09:32 -0400
      Re: dict to boolean expression, how to? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-01 15:02 +0000
        Re: dict to boolean expression, how to? Terry Reedy <tjreedy@udel.edu> - 2014-08-01 18:16 -0400
    Re: dict to boolean expression, how to? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-08-01 16:50 +0100
      Re: dict to boolean expression, how to? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-01 16:35 +0000
  Re: dict to boolean expression, how to? Alex van der Spek <zdoor@xs4all.nl> - 2014-08-01 14:26 +0000
    Re: dict to boolean expression, how to? marco.nawijn@colosso.nl - 2014-08-01 08:07 -0700
    Re: dict to boolean expression, how to? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-01 15:24 +0000
      Re: dict to boolean expression, how to? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-08-01 17:03 +0100
    Re: dict to boolean expression, how to? Peter Otten <__peter__@web.de> - 2014-08-01 17:44 +0200
      eval [was Re: dict to boolean expression, how to?] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-02 02:59 +0000
        Re: eval [was Re: dict to boolean expression, how to?] Peter Otten <__peter__@web.de> - 2014-08-02 09:43 +0200
        Re: eval [was Re: dict to boolean expression, how to?] Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-08-02 08:59 +0100
        Re: eval [was Re: dict to boolean expression, how to?] Duncan Booth <duncan.booth@invalid.invalid> - 2014-08-05 11:57 +0000
          Re: eval [was Re: dict to boolean expression, how to?] Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-06 02:25 +1000
    Re: dict to boolean expression, how to? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-08-01 17:12 +0100
    Re: dict to boolean expression, how to? Peter Pearson <ppearson@nowhere.invalid> - 2014-08-01 17:00 +0000
  Re: dict to boolean expression, how to? Ned Batchelder <ned@nedbatchelder.com> - 2014-08-01 10:29 -0400

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