Closures
A closure is a function that references variables from outside its own function body. The function definition and its environment are bundled together into a single entity.
Put simply, a closure is just a function that keeps track of some values from the place where it was defined, no matter where it is executed later on.
Example
The concatter() function returns a function called doc_builder (yay higher-order functions!) that has a reference to an enclosed doc value.
def concatter():
doc = ""
def doc_builder(word):
# "nonlocal" tells Python to use the 'doc'
# variable from the enclosing scope
nonlocal doc
doc += word + " "
return doc
return doc_builder
# save the returned 'doc_builder' function
# to the new function 'harry_potter_aggregator'
harry_potter_aggregator = concatter()
harry_potter_aggregator("Mr.")
harry_potter_aggregator("and")
harry_potter_aggregator("Mrs.")
harry_potter_aggregator("Dursley")
harry_potter_aggregator("of")
harry_potter_aggregator("number")
harry_potter_aggregator("four,")
harry_potter_aggregator("Privet")
print(harry_potter_aggregator("Drive"))
# Mr. and Mrs. Dursley of number four, Privet Drive
When concatter() is called, it creates a new "stateful" function that remembers the value of its internal doc variable. Each successive call to harry_potter_aggregator appends to that same doc.
nonlocal
Python has a keyword called nonlocal that's required to modify a variable from an enclosing scope. Most programming languages don't require this keyword, but Python does.
Assignment
Doc2Doc keeps track of how many words are in a collection of documents.
In other words, it keeps a running total of the count variable within a closure.
Tip
I used .split() to count the number of words in the doc string.