Standard Library Flashcards

Question

enum

Answer 1

(Data Types) Support for enumerations _________________________________ - An enumeration is a set of symbolic names (members) bound to unique, constant values - Members can be compared by identity - Enum can be iterated over - As they represent constants, use UPPER_CASE names Four classes, a unique decorator, and an auto helper function. ``` Enum - base class for creation Also: IntEnum, IntFlag, Flag ``` unique() - decorator ensures only one name is bound to any one value auto() - instances of auto are replaced with an appropriate value for Enum members (starts at 1) - values are chosen by _generate_next_value(), which can be overridden from enum import Enum ``` @unique class Color(Enum): RED = 1 # or auto() GREEN = 2 # auto() BLUE = 3 # auto() # YELLOW = 3 would raise ValueError as it is a duplicate ``` Iteration: Use special attribute __members__, a read-only ordered mapping of names to members - for name, member in Enum.__members__.items() Comparison: - By identity (Color.RED is Color.RED => True) - NOT by ordered comparison (Color.RED < Color.BLUE) - Equality comparisons are defined (Color.RED == Color.BLUE => False)

Answer 2

(Functional Programming Modules) Higher-order functions and operations on callable objects _________________________________ Higher-order functions act on or return other functions (ice decorators) _________________________________ cache(fxn) - Memoize results ``` @cache def factorial(n): return n * factorial(n-1) if n else 1 ``` factorial(10) => no cached results, makes 11 recursive calls factorial(5) => result is cached, looks up cached result _________________________________ @cached_property - Like property(), with the addition of caching (ie useful for expensive computed properties that are otherwise immutable) - Differs from property() because no setter needs to be defined, value can be cleared by deletion and then can be reset - Requires __dict__ attribute, so it will not work with metaclasses (dict is read-only proxies) or __slots__ (since no dict at all, unless dict is a defined slot) _________________________________ reduce(function, iterable) - Apply function of 2 arguments cumulatively to the items of iterable to reduce it to a single value - Works well with lambda - lambda x, y: x + y (aka return sum of the iterable's elements) _________________________________ update_wrapper(wrapper, wrapped) - Update a wrapper function to look like the wrapped function - Allows access to the original function for introspection wraps(wrapped) - Convenient way to invoke update_wrapper() as a fxn decorator for a wrapper fxn

Answer 3

(File and Directory Access) Object-oriented filesystem paths _________________________________ - Offers classes representing filesystem paths with semantics for different OSs. - Pure paths - w/o I/O - Concrete paths inherit from pure but also provide I/O Basic use: - List subdirectories - List Python source files in the directory tree - Navigating inside a directory tree - Querying path properties - Opening a file

Answer 4

(Data Compression and Archiving) Support for gzip files _________________________________ A simple interface to compress and decompress files like the GNU programs gzip/gunzip would - aka over .gz files - Data compression provided by zlib module - Provides the GzipFile class, open(), compress(), and decompress() convenience functions. - GzipFile class reads/writes gzip-format files so that it looks like an ordinary file object - compress level argument (for functions and class) - 0 no compression, 1-9 being highest compression/slowest time to compress - mtime argument (for compress and class) - last modification time for compression, as an integer open => takes a zn file and opens it as a file object compress/decompress => takes a data argument that it compresses/decompresses returning a bytes object

Answer 5

(Generic Operating System Services) Core tools for working with streams _________________________________ Python's main facilities for dealing with various types of I/O Three main types: text I/O, binary I/O, raw I/O A concrete obj belonging to any of these is called a file object They can have various capabilities: read-only, write-only, read-write. _________________________________ Text I/O - expects/produces str objects (even if the backing store is natively made of bytes) f = open("my file.txt", "r", encoding="utf-9") Binary I/O - (buffered I/O) - expects bytes-like objects and produces bytes objects (no encoding, decoding, or newline translation is performed) - can use 'b' in the mode string, or in front of string literals (makes them bytes) f = open("my file.jpg", "rb") Raw I/O - (unbuffered I/O) - low-level building block for binary and text streams f = open("my file.jpg", "rb", buffering=0) _________________________________ AbstractBaseClasses used to specify the various categories of streams: ``` IOBase - the rest inherit from - doesn't declare read() or write() cuz it will vary, but technically part of the interface RawIOBase BufferedIOBase TextIOBase ``` close() - Flush and close the stream if open readline(size=-1) - Read and return one line from the stream - If size is specified, at most size bytes will be read readlines(hint=-1) - Read and return. list of lines from the stream - Hint can be specified to control the number of lines read - Can loop w/o this, ie for line in file writelines(lines) - Write a list of lines to the stream - Add line separators manually

Answer 6

(Debugging and Profiling) Measure execution time of small code snippets _________________________________ A simple Timer for small bits of code, in the command line and a callable interface Three convenience functions and a class: timeit(stmt='pass', setup='pass', timer=, number=1000000, globals=None) - Create a Timer instance w/ setup code and run its timeit() method w/ number of executions - globals arg specifies a name-since in which to execute repeat(stmt='pass', setup='pass', timer=, repeat=5, number=1000000, globals=None) - Create a Timer instance w/ setup code and run its repeat() method w/ the given repeat count and number of executions default_timer() - The default timer, time.perf_counter() ``` Timer(stmt='pass', setup='pass', timer=, globals=None) - class with the methods you can instantiate itself for repeat timing setup/statement/globals ```