Python
Dictionary Methods and Counting Patterns
setdefault, get with a default and the Counter class turn the most common dictionary jobs - counting, grouping and accumulating - into two or three li...
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Python
Nested Dictionaries and Real Data Structures
Real data arrives as dictionaries inside dictionaries inside lists. Reaching into it safely, walking it and flattening it are skills worth practising...
Python
List and Set Comprehensions
A comprehension builds a collection from an expression and a loop, in one line. It reads left to right in the same order as the loop it replaces.
Python
Dictionary Comprehensions
The same syntax as a list comprehension, with a key and a value separated by a colon. It is the fastest way to build, filter, invert or reshape a mapp...
Python
Defining and Calling Functions
A function packages a piece of work behind a name. def creates it, the call runs it, and return decides what comes back - which is None if you never s...
Python
Function Arguments: Defaults, *args and **kwargs
Positional, keyword, default, variable length and keyword only arguments - five styles, one fixed order, and one famous trap involving mutable default...
Python
Scope and the LEGB Rule
Python resolves a name by looking in four places in a fixed order: local, enclosing, global, built-in. Assignment anywhere in a function makes the nam...
Python
Lambda Functions
A lambda is a small anonymous function written as a single expression. It exists for the places where naming a function would add nothing.
Python
First-Class and Higher-Order Functions
Functions in Python are ordinary objects. They can be stored, passed, returned and built at runtime, and that fact underlies sorting keys, decorators...
Python
Nested Functions and Closures
A closure is an inner function that keeps hold of variables from the function that created it, long after that function has returned.
Python
Recursion: How It Works
A recursive function calls itself on a smaller version of the problem until it reaches a case it can answer directly. Every one needs a base case and...
Python
Recursive Problems and Their Limits
Fibonacci shows why naive recursion can be catastrophically slow, memoisation fixes it in one line, and some problems are worth the stack frames while...