[ Python 常見問題 ] How do I merge two dictionaries in a single expression?

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Question
I have two Python dictionaries, and I want to write a single expression that returns these two dictionaries, merged (i.e. taking the union). The update() method would be what I need, if it returned its result instead of modifying a dictionary in-place:

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1. >>> x = {'a': 1, 'b': 2}  
2. >>> y = {'b': 10, 'c': 11}  
3. >>> z = x.update(y)  
4. >>> print(z)  
5. None  
6. >>> x  
7. {'a': 1, 'b': 10, 'c': 11}  

How can I get that final merged dictionary in z, not x?

HowTo
For dictionaries x and y, z becomes a shallowly merged dictionary with values from y replacing those from x.

In Python 3.9.0 or greater (released 17 October 2020):
PEP-584, discussed here, was implemented and provides the simplest method:

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1. >>> x = {'a': 1, 'b': 2}  
2. >>> y = {'b:': 10, 'c': 11}  
3. >>> z = x | y  
4. >>> z  
5. {'a': 1, 'b': 2, 'b:': 10, 'c': 11}  

In Python 3.5 or greater:

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1. >>> x = {'a': 1, 'b': 2}  
2. >>> y = {'b:': 10, 'c': 11}  
3. >>> z = {**x, **y}  
4. >>> z  
5. {'a': 1, 'b': 2, 'b:': 10, 'c': 11}  

In Python 2, (or 3.4 or lower) write a function:

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1. def merge_two_dicts(x, y):  
2.     z = x.copy()   # start with x's keys and values  
3.     z.update(y)    # modifies z with y's keys and values & returns None  
4.     return z  
5.   
6. x = {'a': 1, 'b': 2}  
7. y = {'b:': 10, 'c': 11}  
8. z = merge_two_dicts(x, y)