Question

I have the following dictionary:

dic = {"a": "first", "b": "second"}

and it’s ok, when I do the following:

print dic.get("a")
print dic.get("a", "asd")
print dic.get("a", dic.get("c"))

but when I use this method like this:

print dic.get("a", dic.get("c").split(" ",1)[0])

I receive the following error:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'split'

I dont’t understand the last case. The second argument calculated (dic.get(“c”) should be None – it’s ok), but there is a key “a” in dictionary and first argument shouldn’t fire calculating of the second argument.

How I can fix this? And why it happened?

TIA!

Answer 1

As others have explained, Python (like most other languages outside the functional family) evaluates all arguments of a function before calling it. Thus, dic.get("c") is None when key “c” doesn’t exist in the dictionary and None has no .split() method, and this evaluation happens regardless of whether (and in fact before) the get succeeds or fails.

Instead, use a short-circuiting Boolean operator or a conditional expression.

# if dic.get["a"] is always truthy when it exists
dic.get("a") or dic.get("c", "").split(" ", 1)[0]

# if dic["a"] could be non-truthy, e.g. empty string
dic["a"] if "a" in dic else dic.get("c", "").split(" ", 1)[0]