Concatenate function arguments with "AND" string if they're not None.

Question

Basically, if the function parameter is None, I ignore it, else I concatenate all arguments like arg1:arg1_value AND arg2:arg2_value AND arg4:arg4_value considering arg3==None when the function got called.

def _get_query_string_for_get_config(self, host, app_name, instance, config_source, config_name):
 query_string = ""
 if host is not None:
 query_string += "host:{} AND ".format(host)
 if app_name is not None:
 query_string += "app_name:{} AND ".format(app_name)
 if instance is not None:
 query_string += "instance:{} AND ".format(instance)
 if config_source is not None:
 query_string += "config_source:{} AND ".format(config_source)
 if config_name is not None:
 query_string += "config_name:{}".format(config_name)
 return query_string

• Richard Neumann: Consider renaming the method as its name is awfully verbose.
• Abhijeet Rastogi: Sure. Thanks @RichardNeumann for the input.
• Richard Neumann: Additionally, if you want to specify what the method is doing, consider adding a docstring.

Answer 1

Concatenating strings with a common connector (such as ' AND ' here) is most idiomatically done using join. So the idea should be to:

return ' AND '.join(<some list with the right `text:value` formatting>)

The easiest way to get such list should be to pair the text part and the associated value using zip and then use a list comprehension to filter out undesired variables:

parameters = zip(
 ('host', 'app_name', 'instance', 'config_source', 'config_name'),
 (host, app_name, instance, config_source, config_name)
)
query = ['{}:{}'.format(text, value) for text, value in parameters if value is not None]
return ' AND '.join(query)

You can also simplify a bit by turning the list-comprehension into a generator expression and feeding it directly to join.

Lastly, the self parameter suggest that you are using this as a method of a class. But since you are not using self in the method, you should either make it a regular function or a saticmethod:

@staticmethod
def _get_query_string_for_get_config(host, app_name, instance, config_source, config_name):
 parameters = zip(
 ('host', 'app_name', 'instance', 'config_source', 'config_name'),
 (host, app_name, instance, config_source, config_name)
 )
 return ' AND '.join(
 '{}:{}'.format(text, value)
 for text, value in parameters
 if value is not None)

Answer 2

Not sure if you even need a function for this. Just make a dictionary with all these parameters and then you can do:

' AND '.join('{}:{}'.format(key, value) for key, value in my_dict.iteritems())

• Richard Neumann: This will not preserve the order of the arguments.
• Alex: @RichardNeumann yes, but since this is a query, order of and filter does not matter.
• Graipher: @Richard And if it is really needed (because one variable is almost always False and you want to use short circuit evaluation) you can just make my_dict a collections.OrderedDict.