Python reference

Each building block has a short explanation, an example with its exact output, and the mistakes people make most.

• print()Show values on the screen. Several values are separated by a space unless you change sep.
• input()Ask the user to type something. The answer always comes back as text.
• Variables and assignmentA variable is a name for a value. = stores a value, and the right side is worked out first.
• f-stringsPut variables and calculations inside text. Start the string with f and use curly brackets.
• len()Count the characters in a string or the items in a list, dictionary or other collection.
• range()A sequence of whole numbers for loops. It starts at 0 and stops before the end.
• if, elif and elseRun code only when a condition is true. Python runs the first branch that matches.
• for loopRepeat code for each item of a sequence, such as a list, a string or a range.
• while loopRepeat code as long as a condition is true. Use it when you do not know the number of turns.
• List methods: append and removeLists can change. append adds to the end, and remove takes out the first matching item.
• Dictionaries and dict.get()Look values up by key. get() returns a default instead of an error when the key is missing.
• def and returnDefine a function with def. return hands a value back to the caller.
• try and exceptHandle an error instead of crashing. Code in try runs; if it raises the named error, except runs.
• classA blueprint for objects that bundle data (attributes) and actions (methods).
• json.dumps() and json.loads()Turn data into JSON text to save or send it, and back into Python data.
• is versus ==is asks whether two names are the same object. == asks whether their values match. Use is None for a missing value.
• TuplesA tuple is an ordered group that cannot change. Use it for fixed records such as a coordinate pair.
• SetsA set keeps unique items and answers fast membership questions. {} is an empty dict; the empty set is set().
• **kwargsExtra named arguments are collected into a dictionary. Read them with .get(key, default).
• yieldyield turns a function into a generator: it hands out one value, pauses, and continues when asked again.
• super()Call the parent method from a subclass. Write the extra behaviour after (or around) that call.
• matchChoose a branch from one value. case _ is the default and must come last.
• re.searchFind the first match of a pattern. Use a raw string r"..." so backslashes stay backslashes. group(1) is the first pair of brackets.