Conditional expressions in Python Dictionary comprehensions

问题

a = {"hello" : "world", "cat":"bat"}

# Trying to achieve this
# Form a new dictionary only with keys with "hello" and their values
b = {"hello" : "world"}

# This didn't work

b = dict( (key, value) if key == "hello" for (key, value) in a.items())

Any suggestions on how to include a conditional expression in dictionary comprehension to decide if key, value tuple should be included in the new dictionary

回答1:

Move the if at the end:

b = dict( (key, value) for (key, value) in a.items() if key == "hello" )

You can even use dict-comprehension (dict(...) is not one, you are just using the dict factory over a generator expression):

b = { key: value for key, value in a.items() if key == "hello" }

回答2:

You don't need to use dictionary comprehension:

>>> a = {"hello" : "world", "cat":"bat"}
>>> b = {"hello": a["hello"]}
>>> b
{'hello': 'world'}

and dict(...) is not dictionary comprehension.

来源：https://stackoverflow.com/questions/18246827/conditional-expressions-in-python-dictionary-comprehensions