PCPP1 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

__new__ vs __init__

__new__ is a static constructor that creates and returns the instance; __init__ then initializes that instance and must return None. Returning a non-None value from __init__ raises TypeError.

C3 linearization

Python's algorithm for computing a class's method resolution order: depth-first merge of base classes preserving local precedence and removing duplicates. Inspect it via ClassName.__mro__ or ClassName.mro().

super() resolution

Zero-argument super() looks up the next class after the current one in type(self).__mro__, so it follows the actual instance's MRO, not the class where the method was defined.

Data descriptor precedence

A descriptor defining __set__ or __delete__ is a data descriptor and takes precedence over the instance __dict__. A non-data descriptor defining only __get__ is overridden by an instance attribute of the same name.

Abstract base class instantiation

A class inheriting abc.ABC with any method still decorated @abstractmethod cannot be instantiated; Python raises TypeError until every abstract method is overridden in a concrete subclass.

__slots__ effect

Declaring __slots__ as a tuple of names removes the per-instance __dict__ and blocks assignment to undeclared attributes, saving memory. A subclass that omits __slots__ reintroduces __dict__.

__eq__ and __hash__

Defining __eq__ in a class sets __hash__ to None, making instances unhashable. Restore hashability by defining __hash__ consistently with equality, typically hashing the same fields compared.

Name mangling

An identifier with two leading underscores and at most one trailing underscore inside a class body is rewritten to _ClassName__identifier, which prevents accidental overriding in subclasses but is not true privacy.

PEP 8 indentation and lines

Use four spaces per indentation level and no tabs. Limit lines to 79 characters, use two blank lines around top-level definitions and one blank line between methods inside a class.

PEP 8 naming conventions

Modules and functions use snake_case, classes use CapWords, constants use UPPER_CASE. A single leading underscore marks non-public API; a trailing underscore avoids clashes with keywords.

Docstring conventions

PEP 257 specifies triple double quotes, a one-line summary ending with a period, and a blank line before extended description. The string is stored in the object's __doc__ attribute.

Runtime behavior of annotations

PEP 484/526 type annotations are stored in __annotations__ but never enforced at runtime. Forward references need string literals or from __future__ import annotations, resolved by typing.get_type_hints().

Import grouping order

PEP 8 orders imports as standard library, third-party, then local application, each group separated by a blank line. Wildcard imports such as from module import * are discouraged.

Zen of Python principles

PEP 20 aphorisms include explicit is better than implicit, flat is better than nested, and errors should never pass silently unless explicitly silenced. Display them by running import this.

contextlib.contextmanager usage

Decorating a generator function with contextlib.contextmanager turns it into a context manager: code before yield runs on entry, code after yield runs on exit, and exceptions surface at the yield.

is None comparison

PEP 8 requires comparing to None with 'is' or 'is not' rather than ==, because identity testing avoids invoking overloaded equality and is the idiomatic singleton check.

Tkinter mainloop threading rule

Tkinter is not thread-safe: the mainloop and widget calls belong on the main thread. Worker threads should post results to a queue.Queue that the main thread polls with after().

Geometry manager mixing

Within a single container widget you must use only one geometry manager: pack, grid, or place. Mixing pack and grid in the same master causes Tk to hang or raise an error.

grid column weight

grid_columnconfigure(index, weight=n) controls how extra space is distributed among columns; without a positive weight the column does not expand when the window is resized.

after scheduling

widget.after(milliseconds, callback) schedules callback on the Tk event loop and returns an identifier; after_cancel(id) cancels it. It is the usual way to run periodic UI updates safely.

Tkinter variable creation

StringVar, IntVar, and BooleanVar need an existing Tk root (or explicit master); creating one before any root exists raises a TclError about no default root window.

Event object attributes

A Tkinter event handler receives an event with attributes such as widget, x and y, keysym for key presses, num for mouse buttons, and width/height for Configure resize events.

filedialog return value

filedialog.askopenfilename() returns the chosen path as a string, or an empty string when the user cancels. Code must test for the empty string before opening the file.

ttk versus tk widgets

ttk widgets provide themed, native-looking controls such as ttk.Treeview, ttk.Notebook, and ttk.Progressbar. Classic tk widgets remain available but do not follow the platform theme.

destroy versus forget

widget.destroy() permanently removes a widget and its children; pack_forget() or grid_forget() only unmaps it so it can be re-added later with the same geometry options.

HTTP verb semantics

GET reads and is safe and idempotent; POST creates and is not idempotent; PUT replaces and is idempotent; PATCH partially updates; DELETE removes and is idempotent.

Key HTTP status codes

200 OK, 201 Created with a Location header, 204 No Content, 400 Bad Request, 401 Unauthorized (missing or bad credentials), 403 Forbidden, 404 Not Found, 429 Too Many Requests, 500 Server Error.

requests json parameter

requests.post(url, json=payload) serializes payload to JSON and sets Content-Type application/json. Using data=payload instead sends form-encoded content unless you set the header yourself.

raise_for_status behavior

response.raise_for_status() raises requests.exceptions.HTTPError when the status code is 4xx or 5xx and does nothing for 2xx and 3xx, so call it before parsing the body.

requests timeout default

requests applies no timeout by default, so a slow server can block forever. Pass timeout=seconds or a (connect, read) tuple to bound how long each request may take.

requests.Session reuse

A Session persists cookies and headers and reuses TCP connections across requests to the same host, which is faster for repeated calls. Close it, ideally with a with statement.

REST pagination patterns

Common schemes are limit and offset parameters, page and page_size parameters, or a Link response header with rel='next'. Clients loop until a page returns no items or no next link.