pycutils 0.0.4

A list of python utilities

Some personal Python utilities. Only uses standard library.

Gotchas

Dictionaries

What is the difference between

dict1 = dict(zip(["a", "b", "c"], [[]] * 3))

and

dict2 = {key:[] for key in ["a", "b", "c"]}

When you print them, both return

{'a': [], 'b': [], 'c': []}

But try, for example,

dict1["a"].append(1)
print(dict1)
dict2["a"].append(1)
print(dict2)

Surprisingly, you get

dict1 = {'a': [1], 'b': [1], 'c': [1]}
dict2 = {'a': [1], 'b': [], 'c': []}

What happened here? The empty lists in dict1 are actually the same list, so when you change one, you change all of them. You can run into the same trap if you initialize a dictionary with

dict.fromkeys(["a", "b", "c"], [])

or if you pass in a mutable object as a default argument to a function.