Python/Django Concatenate a string depending on whether that string exists

Question

I'm creating a property on a Django model called "address". I want address to consist of the concatenation of a number of fields I have on my model. The problem is that not all instances of this model will have values for all of these fields. So, I want to concatenate only those fields that have values.

What is the best/most Pythonic way to do this?

Here are the relevant fields from the model:

house = models.IntegerField('House Number', null=True, blank=True)
suf = models.CharField('House Number Suffix', max_length=1, null=True, blank=True)
unit = models.CharField('Address Unit', max_length=7, null=True, blank=True)
stex = models.IntegerField('Address Extention', null=True, blank=True)
stdir = models.CharField('Street Direction', max_length=254, null=True, blank=True)
stnam = models.CharField('Street Name', max_length=30, null=True, blank=True)
stdes = models.CharField('Street Designation', max_length=3, null=True, blank=True)
stdessuf = models.CharField('Street Designation Suffix',max_length=1, null=True, blank=True)

I could just do something like this:

def _get_address(self):
    return "%s %s %s %s %s %s %s %s" % (self.house, self.suf, self.unit, self.stex, self.stdir, self.stname, self.stdes, self.stdessuf)

but then there would be extra blank spaces in the result.

I could do a series of if statements and concatenate within each, but that seems ugly.

What's the best way to handle this situation?

Thanks.

jdi · Accepted Answer · 2012-07-01 22:00:47Z

4

parts = (1, 'a', 'b', 2, 'c', 'd', None, 'f')
# parts = (self.house, self.suf, self.unit, 
#           self.stex, self.stdir, self.stname, 
#           self.stdes, self.stdessuf)
' '.join(str(part) for part in parts if part is not None)
# '1 a b 2 c d e f'

You just do a list comp, and check if any value is not None, and also convert them to a string so they join properly regardless of type.

edited Jul 1, 2012 at 22:00

answered Jul 1, 2012 at 21:14

jdi

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7 Comments

Douglas Meehan Over a year ago

Thanks for the 'not None' addition

jdi Over a year ago

@DouglasMeehan: Sure. Also if you want to save on creating a temp list every single time you call the method, you could save the fields in a list as a class attribute, since its not going to change between instances.

Douglas Meehan Over a year ago

What's better in this situation? tuple or list?

Douglas Meehan Over a year ago

So i would create another property which would be the list of these fields and then reference that property in the address property?

jdi Over a year ago

A list has a slight overhead over a tuple. But use a tuple since you aren't going to modify it at all. And ya you could create another class attribute at the bottom after the fields, then reference is in your method with self._address_fields

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Maksym Polshcha · Accepted Answer · 2012-07-01 21:17:56Z

2

" ".join(filter(None, 
               [self.house, self.suf, self.unit, 
                self.stex, self.stdir, self.stname, 
                self.stdes, self.stdessuf]))

answered Jul 1, 2012 at 21:17

Maksym Polshcha

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1 Comment

jdi Over a year ago

This breaks if the field is not a string.

cberner · Accepted Answer · 2012-07-01 22:15:09Z

2

I'd use a generator to get just the non-Falsy values, and then join them. For example:

parts = (self.house, self.suf, self.unit, self.stex, self.stdir, self.stname, self.stdes, self.stdessuf)
return " ".join(str(s) for s in parts if s is not None)

edited Jul 1, 2012 at 22:15

answered Jul 1, 2012 at 21:12

cberner

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2 Comments

jdi Over a year ago

You need to check for None, and not a Falsey value. None is not the same as 0 which is a valid number

Douglas Meehan Over a year ago

so in that case, I could do parts = (self.house, self.suf, self.unit, self.stex, self.stdir, self.stname, self.stdes, self.stdessuf) return " ".join(str(s) for s in parts if s is not None)?

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