I get that they have some performance enhancements over for loops but unless you're being forced to really optimize your performance, my experience is that the value of list comprehensions is more that, when use correctly, they can be the better way of communicating what you're doing because of the value you gain from keeping things more concise. They were still bad list comprehensions, but the list comprehensions did start to become less horrible as time went on before that project I'd just never had a reason to deal with list comprehensions. Introduction to Programming with Python (from Microsoft Virtual Academy).
#Combine three dictionaries python code
Also note that our code is general: it makes no presumptions about the dictionaries’ keys - they need not be strings nor of any other particular type. This, along with a good docstring, makes our code easy to read, understand, and debug. Note the use of simple and descriptive variables (e.g. we use the variable name key when we iterate over the keys of a dictionary). Parameters - dict1 : Dict dict2 : Dict Returns - Dict The merged dictionary """ merged = dict ( dict1 ) for key in dict2 : if key not in merged or dict2 > merged : merged = dict2 return merged Thus calling this function will mutate (change) the state of dict1, as demonstrated here:ĭef simple_merge_max_mappings ( dict1, dict2 ): """ Merges two dictionaries based on the largest value in a given mapping. Recall that dictionaries are mutable objects and that the statement merged = dict1 simply assigns a variable that references dict1 rather than creating a new copy of the dictionary. The problem with our function is that we inadvertently merge dict2 into dict1, rather than merging the two dictionaries into a new dictionary. Merged is initialized to have the same mappings as dict1, this is a correct algorithm for merging our two dictionaries based on max-value. We then set a key-value from dict2 mapping in merged if that key doesn’t exist in merged or if the value is larger than the one stored in existing mapping. Thus for key in dict2 loops over every key in dict2. Recall that iterating over a dictionary will produce each of its keys one-by-one. Let’s first see what this function does right. Def buggy_merge_max_mappings ( dict1, dict2 ): # create the output dictionary, which contains all # the mappings from `dict1` merged = dict1 # populate `merged` with the mappings in dict2 if: # - the key doesn't exist in `merged` # - the value in dict2 is larger for key in dict2 : if key not in merged or dict2 > merged : merged = dict2 return merged
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