SQLAlchemy code for calling parameters

Question

I'm using the following function to process API parameters for an specific endpoint. Using SQLAlchemy and Flask, I want to get feedback in terms of simplifying calling SQL code and nested statements.

date_filter = r'\d{4}-((1[0-2]|[1-9])|0[1-9])-((3[01]|[12][0-9]|[1-9])|0[1-9])'
def bits_filter(request):
 """
 :param request:
 :return:(Model.Bits) List of bits
 """
 # Get parameters.
 log.info('/bits request: %s', request)
 # Date parameters.
 date_value = request.args.get('date')
 # Bit or newsletter.
 bit_type_value = request.args.get('bit_type')
 # Process parameters.
 date_value = date_value or settings.DATE_VALUE
 bit_type = bit_type_value or settings.BIT_TYPE 
 log.info('bit_type: %r', bit_type)
 if date_value and isinstance(date_value, unicode):
 # Bit type.
 if date_value.lower() == u'latest':
 return Model.Bits.query.filter(Model.Bits.bit_type == bit_type).order_by(
 Model.Bits.published.desc()).limit(settings.max_bits).all()
 # Dated format.
 elif re.match(date_filter, date_value):
 return Model.Bits.query.filter(
 Model.Bits.bit_type == bit_type & Model.Bits.published == date_value).limit(
 settings.max_bits).all()
 else:
 log.error('Invalid date value parameter: %r', date_value)
 return None 
 else:
 return Model.Bits.query.filter(Model.Bits.bit_type == bit_type).order_by(
 Model.Bits.published.desc()).limit(settings.max_bits).all()

Answer 1

I would re-structure code to improve on readability and avoiding code duplication while decreasing the nestedness depth level - remember that "Flat is better than nested".

I would probably first fail fast if date value is given and has an invalid format. Then, I would define a queryset variable which will help us keep the common parts of the query while building it.

Also, you may pre-compile the regular expression and define it as a proper Python constant - in upper case with underscores.

Something along these lines:

# fail fast if the date value is invalid
if date_value and not DATE_FILTER_PATTERN.match(date_value):
 log.error('Invalid date value parameter: %r', date_value)
 return None
queryset = Model.Bits.query.filter(Model.Bits.bit_type == bit_type)
if not date_value or date_value.lower() == 'latest':
 queryset = queryset.order_by(Model.Bits.published.desc())
else:
 queryset = queryset.filter(Model.Bits.published == date_value)
return queryset.limit(settings.max_bits).all()

where DATE_FILTER_PATTERN is defined as:

DATE_FILTER_PATTERN = re.compile(r'\d{4}-((1[0-2]|[1-9])|0[1-9])-((3[01]|[12][0-9]|[1-9])|0[1-9])')