Only add to a dict if a condition is met

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

One technique I suggest is using the dictionary unpacking operatior for this.

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

Explanation

Basically, if orange is None, then the above dictionary simplifies to

{
    'apple': apple,
    **({})
}

# which results in just
{
    'apple': apple,
} 

Opposite goes with if orange is not None:

{
    'apple': apple,
    **({ "orange": orange })
}

# which results in just
{
    'apple': apple,
    'orange': orange
} 

Readablity is a downside for conditionally adding keys inline. It is possible to create a function that could help mediate the readability issue.

from typing import Callable

def cond_pairs(
        cond: bool, pairs: Callable[[], dict],
) -> dict:
    return pairs() if cond else {}

{
    'apple': apple,
    **cond_pairs(orange, lambda: { 'orange': orange })
}

Pretty old question but here is an alternative using the fact that updating a dict with an empty dict does nothing.

def urlencode_func(apple, orange=None):
    kwargs = locals().items()
    params = dict()
    for key, value in kwargs:
        params.update({} if value is None else {key: value})
    return urllib.urlencode(params)

I did this. Hope this help.

apple = 23
orange = 10
a = {
    'apple' : apple,
    'orange' if orange else None : orange
}

Expected output : {'orange': 10, 'apple': 23}

Although, if orange = None , then there will be a single entry for None:None. For example consider this :

apple = 23
orange = None
a = {
    'apple' : apple,
    'orange' if orange else None : orange
}

Expected Output : {None: None, 'apple': 23}

You can clear None after the assignment:

apple = 'green'
orange = None
dictparams = {
    'apple': apple,
    'orange': orange
}
for k in dictparams.keys():
    if not dictparams[k]:
        del dictparams[k]
params = urllib.urlencode(dictparams)

I find using a generator function to be easier to understand, and flexible enough. It also works with both Python 2 and 3.

def generate_request_items(apple, orange):
    yield "apple", apple
    if orange:
        yield "orange", orange
    # Add additional conditionals and yield statements here


apple = 'green'
orange = None
params = urllib.urlencode(dict(generate_request_items(apple, orange)))

Another valid answer is that you can create you own dict-like container that doesn't store None values.

class MyDict:
    def __init__(self):
        self.container = {}
    def __getitem__(self, key):
        return self.container[key]
    def __setitem__(self, key, value):
        if value != None:
            self.container[key] = value
    def __repr__(self):
        return self.container.__repr__()

a = MyDict()
a['orange'] = 'orange';
a['lemon'] = None

print a

yields:

{'orange': 'orange'}

I really like the neat trick in the answer here: https://stackoverflow.com/a/50311983/3124256

But, it has some pitfalls:

1. Duplicate if tests (repeated for key and value)
2. Pesky None: None entry in the resulting dict

To avoid this, you can do the following:

apple = 23
orange = None
banana = None
a = {
    'apple' if apple else None: apple,
    'orange' if orange else None : orange,
    'banana' if banana else None: banana,
    None: None,
}
del a[None]

Expected Output : {'apple': 23}

Note: the None: None entry ensures two things:

1. The None key will always be present (del won't throw an error)
2. The contents of 'None values' will never exist in the dict (in case you forget to del afterwards)

If you aren't worried about these things, you can leave it out and wrap the del in a try...except (or check if the None key is present before deling). To address number 2 alternatively, you could also put the conditional check on the value (in addition to the key).

You can deal with all optional items using a single condition by using a dictionary comprehension:

apple = "red"
orange = None
dictparams = {
    key: value for key, value in
    {
        "apple": apple,
        "orange": orange
    }.items()
    if value is not None
}

The dictparams result will not contain "orange" in this case, because orange is None:

{'apple': 'red'}

You can add the data and then filter it via a condition afterwards:

data = {
    "foo": None,
    "bar": "1234",
    "baz": None,
}
data = {k: v for k, v in data.items() if v is not None}

would result in:

data = {
    "bar": "1234",
}

See also this question/answers for filter/lambda functions on a dictionary:

How to filter a dictionary according to an arbitrary condition function?

fruits = [("apple", get_apple()), ("orange", get_orange()), ...]

params = urllib.urlencode({ fruit: val for fruit, val in fruits if val is not None })

There is a counter-intuitive but reliable hack, to reuse the other prop name you want to exclude it.

{
    'orange' if orange else 'apple': orange,
    'apple': apple,
}

In this case, the latter 'apple' will override the previous 'apple' effectively removing it. Note that the conditional expressions should go above the real ones.

Building of the answer from kindall, I'm using this to filter out not just None values, but anything that evaluates to False. This suits my scenario as I don't want anything that is empty, False or None.

class DictNoNone(dict):

    def __init__(self, iterable=(), **kwargs):
        for k, v in iterable:
            if v: self[k] = v
        for k, v in kwargs.items():
            if v: self[k] = v

    def __setitem__(self, key, value):
        if not value:
            if key in self:
                del self[key]
        else:
            dict.__setitem__(self, key, value)

A slightly cursed "improvement" to attila's answer would be

@lambda f: lambda *args: dict(f(*args))
def optional_oranges(apple, orange = None):
    yield "apple", apple
    if orange is not None:
        yield "orange", orange

optional_oranges(1) #{'apple': 1}
optional_oranges(1,2) #{'apple': 1, 'orange': 2}