Re: [web2py] Re: field in list test
Thanks Anthony ... I'm sorry say but can not remember for what i needed that code snippet thanks any way.. is good to know that stuff. 2016-09-16 17:02 GMT-04:00 Anthony : > This is a quirk of Field objects. Field inherits from Expression, and the > __eq__ method of Expression returns a Query object (rather than testing for > equality and returning a boolean). So, if you do something like myfield == > some_object, you do not actually get a test of whether myfield is equivalent > to some_object. Instead, you just get a Query object, which apparently > evaluates to True in the code Python uses to check for the existence of an > element in a list. It works this way so you can create DAL queries using the > db.mytable.myfield == some_value syntax. > > If you want to check for a field in a list, consider instead storing the > field names: > > f = db.item.id > fields = [f.name] > f.name in fields > > Or using your current code, you can do: > > any(field is f for field in fields) > > Above, field == f would result in the same problem, but field is f avoids > the creation of the Query object. > > Anthony > > > On Friday, September 16, 2016 at 12:41:28 PM UTC-4, Yoel Benitez Fonseca > wrote: >> >> Does this make sense to you? >> >> In [1]: fields = list() >> >> In [2]: f = db.item.id >> >> In [3]: f in fields >> Out[3]: False >> >> In [4]: fields.append(f) >> >> In [5]: f in fields >> Out[5]: True >> >> In [6]: f = db.item.headline >> >> In [7]: f in fields >> Out[7]: True >> >> I mean, the last value of 'f' is a fields object but a different one. >> >> -- >> Yoel Benítez Fonseca >> http://redevil.cubava.cu/ >> $ python -c "import this" > > -- > Resources: > - http://web2py.com > - http://web2py.com/book (Documentation) > - http://github.com/web2py/web2py (Source code) > - https://code.google.com/p/web2py/issues/list (Report Issues) > --- > You received this message because you are subscribed to the Google Groups > "web2py-users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to web2py+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- Yoel Benítez Fonseca http://redevil.cubava.cu/ $ python -c "import this" -- Resources: - http://web2py.com - http://web2py.com/book (Documentation) - http://github.com/web2py/web2py (Source code) - https://code.google.com/p/web2py/issues/list (Report Issues) --- You received this message because you are subscribed to the Google Groups "web2py-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to web2py+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
[web2py] Re: field in list test
This is a quirk of Field objects. Field inherits from Expression, and the __eq__ method of Expression returns a Query object (rather than testing for equality and returning a boolean). So, if you do something like myfield == some_object, you do not actually get a test of whether myfield is equivalent to some_object. Instead, you just get a Query object, which apparently evaluates to True in the code Python uses to check for the existence of an element in a list. It works this way so you can create DAL queries using the db.mytable.myfield == some_value syntax. If you want to check for a field in a list, consider instead storing the field names: f = db.item.id fields = [f.name] f.name in fields Or using your current code, you can do: any(field is f for field in fields) Above, field == f would result in the same problem, but field is f avoids the creation of the Query object. Anthony On Friday, September 16, 2016 at 12:41:28 PM UTC-4, Yoel Benitez Fonseca wrote: > > Does this make sense to you? > > In [1]: fields = list() > > In [2]: f = db.item.id > > In [3]: f in fields > Out[3]: False > > In [4]: fields.append(f) > > In [5]: f in fields > Out[5]: True > > In [6]: f = db.item.headline > > In [7]: f in fields > Out[7]: True > > I mean, the last value of 'f' is a fields object but a different one. > > -- > Yoel Benítez Fonseca > http://redevil.cubava.cu/ > $ python -c "import this" > -- Resources: - http://web2py.com - http://web2py.com/book (Documentation) - http://github.com/web2py/web2py (Source code) - https://code.google.com/p/web2py/issues/list (Report Issues) --- You received this message because you are subscribed to the Google Groups "web2py-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to web2py+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
[web2py] Re: field in list test
On Friday, September 16, 2016 at 9:41:28 AM UTC-7, Yoel Benitez Fonseca wrote: > > Does this make sense to you? > Yes. > > In [1]: fields = list() > > In [2]: f = db.item.id > > In [3]: f in fields > Out[3]: False > > In [4]: fields.append(f) > > In [5]: f in fields > Out[5]: True > > In [6]: f = db.item.headline > > In [7]: f in fields > Out[7]: True > > I mean, the last value of 'f' is a fields object but a different one. > What you've stored in fields is an object reference; a reference to f, not to its value. What are you trying to do? Dynamic table handling, or forming a list of fields for SQLFORM or ?? /dps -- Resources: - http://web2py.com - http://web2py.com/book (Documentation) - http://github.com/web2py/web2py (Source code) - https://code.google.com/p/web2py/issues/list (Report Issues) --- You received this message because you are subscribed to the Google Groups "web2py-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to web2py+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
