Python's equivalent to null-conditional operator introduced in C# 6

Question

Is there an equivalent in Python to C# null-conditional operator?

System.Text.StringBuilder sb = null;
string s = sb?.ToString(); // No error

Asked By: Andrey Grachev

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Answer 1

Well, the simplest solution would be:

result = None if obj is None else obj.method()

But if you want the exact equivalent having the same thread safety as the C#’s Null-conditional operator, it would be:

obj = 'hello'
temp = obj
result = None if temp is None else temp.split()

The trade off is that the code isn’t really pretty; Also an extra name temp gets added to the namespace.

Another way is:

def getattr_safe(obj, attr):
    return None if obj is None else getattr(obj,attr)

obj = 'hello'
result = getattr_safe(obj,'split')()

Here, trade-off is the function calling overhead, but much clearer code, especially if you are using it multiple times.

Answered By: Tanmay

Answer 2

How about:

s = sb and sb.ToString()

The short circuited Boolean stops if sb is Falsy, else returns the next expression.

Btw, if getting None is important…

sb = ""

#we wont proceed to sb.toString, but the OR will return None here...
s = (sb or None) and sb.toString()

print s, type(s)

output:

None <type 'NoneType'>

There was some over-eager deletion of comments in the past, which I want to replicate here as a caveat:

commenter : Do you really think it is a good idea to do this in Python, a language that prides itself on code readability?

me : No, I don’t. This answers the question – and illustrates some aspects of boolean short-circuiting – but is too "clever" by half. If you have a reason to use this form, do so, but be aware its intent is not crystal clear at first glance.

Answered By: JL Peyret

Answer 3

I wrote this function with your required behavior. An advantage of this over chaining and is that it’s easier to write when it comes to long chains. Heads up this doesn’t work with object keys, only attributes.

def null_conditional(start, *chain):
    current = start
    for c in chain:
        current = getattr(current, c, None)
        if current is None:
            break
    return current

Here’s some tests I ran so you can see how it works

class A(object):
    b = None
    def __init__(self, v):
        self.b = v

class B(object):
    c = None
    def __init__(self, v):
        self.c = v    

a_1 = A(B(2))
a_2 = A(None)
print(null_conditional(a_1, 'b', 'c')) # 2
print(null_conditional(a_1, 'b', 'd')) # None
print(null_conditional(a_2, 'b', 'c')) # None
print(null_conditional(None, 'b')) # None
print(null_conditional(None, None)) # TypeError: attribute name must be string

Answered By: Jad S

Answer 4

There’s a proposal under PEP-505, in the meantime there’s a library for this:

from pymaybe import maybe

print(maybe(None).toString())

Answered By: c z

Python's equivalent to null-conditional operator introduced in C# 6

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