Only add to a dict if a condition is met

Question

I am using urllib.urlencode to build web POST parameters, however there are a few values I only want to be added if a value other than None exists for them.

apple = 'green' orange = 'orange' params = urllib.urlencode({     'apple': apple,     'orange': orange }) 

That works fine, however if I make the orange variable optional, how can I prevent it from being added to the parameters? Something like this (pseudocode):

apple = 'green' orange = None params = urllib.urlencode({     'apple': apple,     if orange: 'orange': orange }) 

I hope this was clear enough, does anyone know how to solve this?

Answer 1

You'll have to add the key separately, after the creating the initial dict:

params = {'apple': apple} if orange is not None:     params['orange'] = orange params = urllib.urlencode(params) 

Python has no syntax to define a key as conditional; you could use a dict comprehension if you already had everything in a sequence:

params = urllib.urlencode({k: v for k, v in (('orange', orange), ('apple', apple)) if v is not None}) 

but that's not very readable.

If you are using Python 3.9 or newer, you could use the new dict merging operator support and a conditional expression:

params = urllib.urlencode(     {'apple': apple} |      ({'orange': orange} if orange is not None else {}) ) 

but I find readability suffers, and so would probably still use a separate if expression:

params = {'apple': apple} if orange is not None:     params |= {'orange': orange} params = urllib.urlencode(params) 

Another option is to use dictionary unpacking, but for a single key that's not all that more readable:

params = urllib.urlencode({     'apple': apple,     **({'orange': orange} if orange is not None else {}) }) 

I personally would never use this, it's too hacky and is not nearly as explicit and clear as using a separate if statement. As the Zen of Python states: Readability counts.

Answer 2

To piggyback on sqreept's answer, here's a subclass of dict that behaves as desired:

class DictNoNone(dict):     def __setitem__(self, key, value):         if key in self or value is not None:             dict.__setitem__(self, key, value)   d = DictNoNone() d["foo"] = None assert "foo" not in d 

This will allow values of existing keys to be changed to None, but assigning None to a key that does not exist is a no-op. If you wanted setting an item to None to remove it from the dictionary if it already exists, you could do this:

def __setitem__(self, key, value):     if value is None:         if key in self:             del self[key]     else:         dict.__setitem__(self, key, value) 

Values of None can get in if you pass them in during construction. If you want to avoid that, add an __init__ method to filter them out:

def __init__(self, iterable=(), **kwargs):     for k, v in iterable:         if v is not None: self[k] = v     for k, v in kwargs.iteritems():         if v is not None: self[k] = v 

You could also make it generic by writing it so you can pass in the desired condition when creating the dictionary:

class DictConditional(dict):     def __init__(self, cond=lambda x: x is not None):         self.cond = cond     def __setitem__(self, key, value):         if key in self or self.cond(value):             dict.__setitem__(self, key, value)  d = DictConditional(lambda x: x != 0) d["foo"] = 0   # should not create key assert "foo" not in d