Aug 14, 2010 · As most answers state setdefault or defaultdict would let you set a default value when a key doesn't exist. However, I would like to point out a small caveat with regard to the use cases of setdefault. When the Python interpreter executes setdefaultit will always evaluate the second argument to the function even if the key exists in the ...

Sep 15, 2011 · variant 1 is by far the slowest because it copies the list every time, variant 2 is the second slowest, variant 3 is the fastest but won't work if you need Python older than 2.5, and variant 4 is just slightly slower than variant 3.

Aug 23, 2015 · No Python operation does implicit copying. Ever. Implicit copying is evil, as far as Python is concerned. It's literals that create objects. Every time setdefault is called, it evaluates both its arguments. When it evaluates its second argument ([]), a new list is created. It's completely the same as a = [].

Feb 4, 2012 · I know you can use setdefault(key, value) to set default value for a given key, but is there a way to set default values of all keys to some value after creating a dict ? Put it another way, I want the dict to return the specified default value for every key I didn't yet set.

because, setdefault will first look for the key elem in it and it found something, then it will return the value corresponding to it. And if it doesn't then it will create key elem and use the second argument passed to it as the value and then return then value.

tl;dr setdefault() has a return value. (It returns the value of the key that was just inserted.) e.g. >>> res = curr.setdefault(a_key, a_value) >>> print(res) a_value I was also confused at first, thinking setdefault has no return value and thus >>> res = curr.setdefault(a_key, a_value) >>> print(res) {a_key: a_value} # incorrect

It would make sense that defaultdict is faster that dict.setdefault() since the former sets its default for the entire dict at creation time, whereas setdefault() does it per element when it is read. One reason to use setdefault is when the default you assign is based on the key (or something) rather than a generic default for the entire dict.

May 31, 2011 · This provides the exact same functionality as the dict type except that it overrides the setdefault() method and will take a dictionary containing one or more items. You can set the defaults at creation. This is just a personal preference. As I understand all that dict.setdefault() does is set the items which haven't been set yet.

Sep 15, 2014 · In contrast, using root.setdefault('a', {}) without reassigning its return value to root works:. tree = {} def add_to_tree(root, value_string): """Given a string of characters `value_string`, create or update a series of dictionaries where the value at each level is a dictionary of the characters that have been seen following the current character.

Jul 8, 2009 · The first form is how you implement that in Python 2. The idiom is so important that in Python 3 it now has special supporting syntax: every argument after a single * in the def signature is keyword-only, that is, cannot be passed as a positional argument, but only as a named one. So in Python 3 you could code the above as: