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
q = d[x] if x in d else (d[x] = {})
but this gives the syntax error. What am I missing?
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6 Answers
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().
Comments
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.
Comments
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.
Comments
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
>>> {}
Comments
That's what setdefault() is for:
q = d.setdefault(x, {})
It does exactly what you want:
- Return
d[x]ifxis a key ind - Assign
{}tod[x]ifxis not yet a key and return that
Comments
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.
2 Comments
georg
Well, this is an old question... Unfortunately, your answer is not correct - I was looking for a way to return an empty dict and to assign it to
d[x] at the same time - something your code doesn't do (please check other answers). Still, +1 for the effort and welcome to SO!
user2027294
Thanks for pointing out my mistake. I edited my answer to reflect its incomplete nature. Also, thanks for the warm welcome!