python ternary operator with assignment

Question

I am new to python. I\'m trying to write this

if x not in d:
    d[x] = {}
q = d[x]

in a more compact way using the ternary operator

Answer 1

That's what setdefault() is for:

q = d.setdefault(x, {})

It does exactly what you want:

• Return d[x] if x is a key in d
• Assign {} to d[x] if x is not yet a key and return that

Answer 2

In Python, assignments cannot occur in expressions, therefore you can't write code like a = (b = c).

You're looking for setdefault:

q = d.setdefault(x, {})

Alternative, use a defaultdict.

Answer 3

The conditional operator in Python is used for expressions only, but assignments are statements. You can use

q = d.setdefault(x, {})

to get the desired effect in this case. See also the documentation of dict.setdefault().

Answer 4

The reason that else (d[x] = {}) is a syntax error is that in Python, assignment is a statement. But the conditional operator expects expressions, and while every expression can be a statement, not every statement is an expression.

Answer 5

For those interested in the ternary operator (also called a conditional expression), here is a way to use it to accomplish half of the original goal:

q = d[x] if x in d else {}

The conditional expression, of the form x if C else y, will evaluate and return the value of either x or y depending on the condition C. The result will then be assigned to q.

Answer 6

You can also use Python's dictionary get() method

q = d.get(x, {})

Explanation:

The get() method returns the value for the specified key if key is in a dictionary.

The syntax of get() is:

dict.get(key[, value]) 

get() Parameters

The get() method takes maximum of two parameters:

key - key to be searched in the dictionary

value (optional) - Value to be returned if the key is not found. The default value is None.

Return Value from get()

The get() method returns:

• the value for the specified key if key is in dictionary.
• None if the key is not found and value is not specified.
• value if the key is not found and value is specified.

Another option is to use defaultdict

from collections import defaultdict

d = defaultdict(dict)
q = d[x]

>>> q
>>> {}