Merge two hashes/dictionaries in Ruby and Python.

merge.rb

h1 = { "a" => 100, "b" => 200 }
h2 = { "b" => 254, "c" => 300 }
h3 = h1.merge(h2)   
h3 #=> {"a"=>100, "b"=>254, "c"=>300}
h1 #=> { "a" => 100, "b" => 200 }

update.py

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

you may be amused to learn that this is the first hit on google for

python update ruby merge

And this one

ruby merge dictionaries

An important distinction is that in Python when using x.update() it will update x, e.g. changes the instance you're calling it on. In Ruby this is not the case, call x.merge(y) doesn't change x, instead the method returns a new Hash.

Not trolling here but to be honest I prefer Ruby's behaviour as it doesn't have a side effect. Bear in mind you can achieve Python's behaviour (update the instance) by calling #merge! instead of #merge.

Today I discovered that there is a nice way to "merge" dictionaries in Python:

{ **x, **y }

This is very useful because you can get assign this result to a new dictionary or update the existing one:

x = {'a':1, 'b': 2}
y = {'b':10, 'c': 11}
z = {**x, **y}  # This creates a new dictionary and assign to z. Similar to Ruby's `z = x.merge(y)`
x = {**x, **y}  # This updates `x` with the result of the merge. Similar to Ruby's `x.merge!(y)`

That's only valid syntax in Python 3.5. In Python 2, you could do

x = {'a': 1}
y = {'b': 2}
z = dict(x, **y)

x = {**x, **y} # not the same as
x.update(y)  
# because a new dict is constructed and assigned to x

x = {'x': 'xxxx'}

y = {'y': 'yyyy'}

id(x), id(y)

(140215338406176, 140215338368816)

x = {**x, **y}; x

{'x': 'xxxx', 'y': 'yyyy'}

id(x)

140215338405960