Module operator provides functions itemgetter() and mul() that offer the same functionality as lambda expressions above.
<list>.insert(<int>, <el>) # Inserts item at index and moves the rest to the right.<el>=<list>.pop([<int>]) # Returns and removes item at index or from the end.<int>=<list>.count(<el>) # Returns number of occurrences. Also works on strings.<int>=<list>.index(<el>) # Returns index of the first occurrence or raises ValueError.<list>.remove(<el>) # Removes first occurrence of the item or raises ValueError.<list>.clear() # Removes all items. Also works on dictionary and set.
Dictionary
<view>=<dict>.keys() # Coll. of keys that reflects changes.<view>=<dict>.values() # Coll. of values that reflects changes.<view>=<dict>.items() # Coll. of key-value tuples that reflects chgs.
value=<dict>.get(key, default=None) # Returns default if key is missing.value=<dict>.setdefault(key, default=None) # Returns and writes default if key is missing.<dict>=collections.defaultdict(<type>) # Creates a dict with default value of type.<dict>=collections.defaultdict(lambda: 1) # Creates a dict with default value 1.
<dict>=dict(<collection>) # Creates a dict from coll. of key-value pairs.<dict>=dict(zip(keys, values)) # Creates a dict from two collections.<dict>=dict.fromkeys(keys [, value]) # Creates a dict from collection of keys.
<dict>.update(<dict>) # Adds items. Replaces ones with matching keys.value=<dict>.pop(key) # Removes item or raises KeyError.
{kfork, vin<dict>.items() ifv==value} # Returns set of keys that point to the value.
{k: vfork, vin<dict>.items() ifkinkeys} # Returns a dictionary, filtered by keys.
<iter>=iter(<collection>) # `iter()` returns unmodified iterator.<iter>=iter(<function>, to_exclusive) # A sequence of return values until 'to_exclusive'.<el>=next(<iter> [, default]) # Raises StopIteration or returns 'default' on end.<list>=list(<iter>) # Returns a list of iterator's remaining elements.
Each abstract base class specifies a set of virtual subclasses. These classes are then recognized by isinstance() and issubclass() as subclasses of the ABC, although they are really not. ABC can also manually decide whether or not a specific class is its virtual subclass, usually based on which methods the class has implemented. For instance, Iterable ABC looks for method iter() while Collection ABC looks for methods iter(), contains() and len().
<str>=<str>.strip() # Strips all whitespace characters from both ends.<str>=<str>.strip('') # Strips all passed characters from both ends.
<list>=<str>.split() # Splits on one or more whitespace characters.<list>=<str>.split(sep=None, maxsplit=-1) # Splits on 'sep' str at most 'maxsplit' times.<list>=<str>.splitlines(keepends=False) # Splits on [\n\r\f\v\x1c\x1d\x1e\x85] and '\r\n'.<str>=<str>.join(<coll_of_strings>) # Joins elements using string as a separator.
<bool>=<sub_str>in<str># Checks if string contains a substring.<bool>=<str>.startswith(<sub_str>) # Pass tuple of strings for multiple options.<bool>=<str>.endswith(<sub_str>) # Pass tuple of strings for multiple options.<int>=<str>.find(<sub_str>) # Returns start index of the first match or -1.<int>=<str>.index(<sub_str>) # Same but raises ValueError if missing.
<str>=<str>.replace(old, new [, count]) # Replaces 'old' with 'new' at most 'count' times.<str>=<str>.translate(<table>) # Use `str.maketrans()` to generate table.
<str>=chr(<int>) # Converts int to Unicode char.<int>=ord(<str>) # Converts Unicode char to int.
Also: 'lstrip()', 'rstrip()'.
Also: 'lower()', 'upper()', 'capitalize()' and 'title()'.
importre<str>=re.sub(<regex>, new, text, count=0) # Substitutes all occurrences with 'new'.<list>=re.findall(<regex>, text) # Returns all occurrences as strings.<list>=re.split(<regex>, text, maxsplit=0) # Use brackets in regex to include the matches.<Match>=re.search(<regex>, text) # Searches for first occurrence of the pattern.<Match>=re.match(<regex>, text) # Searches only at the beginning of the text.<iter>=re.finditer(<regex>, text) # Returns all occurrences as match objects.
Search() and match() return None if they can't find a match.
Argument 'flags=re.IGNORECASE' can be used with all functions.
Argument 'flags=re.MULTILINE' makes '^' and '$' match the start/end of each line.
Argument 'flags=re.DOTALL' makes dot also accept the '\n'.
Use r'\1' or '\\1' for backreference.
Add '?' after an operator to make it non-greedy.
Match Object
<str>=<Match>.group() # Returns the whole match. Also group(0).<str>=<Match>.group(1) # Returns part in the first bracket.<tuple>=<Match>.groups() # Returns all bracketed parts.<int>=<Match>.start() # Returns start index of the match.<int>=<Match>.end() # Returns exclusive end index of the match.
Special Sequences
By default, decimal characters, alphanumerics and whitespaces from all alphabets are matched unless 'flags=re.ASCII' argument is used.
As shown below, it restricts special sequence matches to '[\x00-\x7f]' and prevents '\s' from accepting '[\x1c\x1d\x1e\x1f]'.
When both rounding up and rounding down are possible, the one that returns result with even last digit is chosen. That makes '{6.5:.0f}' a '6' and '{7.5:.0f}' an '8'.
fromrandomimportrandom, randint, choice, shuffle, gauss, seed<float>=random() # A float inside [0, 1).<int>=randint(from_inc, to_inc) # An int inside [from_inc, to_inc].<el>=choice(<list>) # Keeps the list intact.
<int>=<int>&<int># And<int>=<int>|<int># Or<int>=<int>^<int># Xor (0 if both bits equal)<int>=<int><<n_bits# Left shift (>> for right)<int>=~<int># Not (also: - - 1)
Combinatorics
Every function returns an iterator.
If you want to print the iterator, you need to pass it to the list() function first!
>>>product('abc', 'abc') # a b c
[('a', 'a'), ('a', 'b'), ('a', 'c'), # a x x x
('b', 'a'), ('b', 'b'), ('b', 'c'), # b x x x
('c', 'a'), ('c', 'b'), ('c', 'c')] # c x x x
>>>combinations('abc', 2) # a b c
[('a', 'b'), ('a', 'c'), # a . x x
('b', 'c')] # b . . x
>>>combinations_with_replacement('abc', 2) # a b c
[('a', 'a'), ('a', 'b'), ('a', 'c'), # a x x x
('b', 'b'), ('b', 'c'), # b . x x
('c', 'c')] # c . . x
>>>permutations('abc', 2) # a b c
[('a', 'b'), ('a', 'c'), # a . x x
('b', 'a'), ('b', 'c'), # b x . x
('c', 'a'), ('c', 'b')] # c x x .
Datetime
Module 'datetime' provides 'date' , 'time' , 'datetime' and 'timedelta'
classes. All are immutable and hashable.
Time and datetime objects can be 'aware' , meaning they have defined timezone, or 'naive' , meaning they don't.
If object is naive, it is presumed to be in the system's timezone.
fromdatetimeimportdate, time, datetime, timedeltafromdateutil.tzimportUTC, tzlocal, gettz, datetime_exists, resolve_imaginary
Use '.weekday()' to get the day of the week (Mon == 0).
'fold=1' means the second pass in case of time jumping back for one hour.
' = resolve_imaginary()' fixes DTs that fall into the missing hour.
Now
<D/DTn>=D/DT.today() # Current local date or naive datetime.<DTn>=DT.utcnow() # Naive datetime from current UTC time.<DTa>=DT.now(<tzinfo>) # Aware datetime from current tz time.
To extract time use '.time()', '.time()' or '.timetz()'.
Timezone
<tzinfo>=UTC# UTC timezone. London without DST.<tzinfo>=tzlocal() # Local timezone. Also gettz().<tzinfo>=gettz('/') # 'Continent/City_Name' timezone or None.<DTa>=<DT>.astimezone(<tzinfo>) # Datetime, converted to the passed timezone.<Ta/DTa>=<T/DT>.replace(tzinfo=<tzinfo>) # Unconverted object with a new timezone.
Encode
<D/T/DT>=D/T/DT.fromisoformat('') # Object from ISO string. Raises ValueError.<DT>=DT.strptime(<str>, '') # Datetime from str, according to format.<D/DTn>=D/DT.fromordinal(<int>) # D/DTn from days since the Gregorian NYE 1.<DTn>=DT.fromtimestamp(<real>) # Local time DTn from seconds since the Epoch.<DTa>=DT.fromtimestamp(<real>, <tz.>) # Aware datetime from seconds since the Epoch.
ISO strings come in following forms: 'YYYY-MM-DD', 'HH:MM:SS.ffffff[±]', or both separated by an arbitrary character. Offset is formatted as: 'HH:MM'.
Epoch on Unix systems is: '1970-01-01 00:00 UTC', '1970-01-01 01:00 CET', ...
Decode
<str>=<D/T/DT>.isoformat(sep='T') # Also timespec='auto/hours/minutes/seconds'.<str>=<D/T/DT>.strftime('') # Custom string representation.<int>=<D/DT>.toordinal() # Days since Gregorian NYE 1, ignoring time and tz.<float>=<DTn>.timestamp() # Seconds since the Epoch, from DTn in local tz.<float>=<DTa>.timestamp() # Seconds since the Epoch, from DTa.
Format
>>>fromdatetimeimportdatetime>>>dt=datetime.strptime('2015-05-14 23:39:00.00 +0200', '%Y-%m-%d %H:%M:%S.%f %z')
>>>dt.strftime("%A, %dth of %B '%y, %I:%M%p %Z")
"Thursday, 14th of May '15, 11:39PM UTC+02:00"
When parsing, '%z' also accepts '±HH:MM'.
For abbreviated weekday and month use '%a' and '%b'.
Arithmetics
<D/DT>=<D/DT> ± <TD># Returned datetime can fall into missing hour.<TD>=<D/DTn>-<D/DTn># Returns the difference, ignoring time jumps.<TD>=<DTa>-<DTa># Ignores time jumps if they share tzinfo object.<TD>=<DT_UTC>-<DT_UTC># Convert DTs to UTC to get the actual delta.
A duck type is an implicit type that prescribes a set of special methods. Any object that has those methods defined is considered a member of that duck type.
Comparable
If eq() method is not overridden, it returns 'id(self) == id(other)', which is the same as 'self is other'.
That means all objects compare not equal by default.
Only the left side object has eq() method called, unless it returns NotImplemented, in which case the right object is consulted.
Extending it generates iter(), contains(), reversed(), index() and count().
Unlike 'abc.Iterable' and 'abc.Collection', it is not a duck type. That is why 'issubclass(MySequence, abc.Sequence)' would return False even if MySequence had all the methods defined.
If there are no numeric values before auto(), it returns 1.
Otherwise it returns an increment of the last numeric value.
<member>=<enum>.<member_name># Returns a member.<member>=<enum>[''] # Returns a member or raises KeyError.<member>=<enum>(<value>) # Returns a member or raises ValueError.<str>=<member>.name# Returns member's name.<obj>=<member>.value# Returns member's value.
BaseException
+-- SystemExit # Raised by the sys.exit() function.
+-- KeyboardInterrupt # Raised when the user hits the interrupt key (ctrl-c).
+-- Exception # User-defined exceptions should be derived from this class.
+-- ArithmeticError # Base class for arithmetic errors.
| +-- ZeroDivisionError # Raised when dividing by zero.
+-- AttributeError # Raised when an attribute is missing.
+-- EOFError # Raised by input() when it hits end-of-file condition.
+-- LookupError # Raised when a look-up on a collection fails.
| +-- IndexError # Raised when a sequence index is out of range.
| +-- KeyError # Raised when a dictionary key or set element is not found.
+-- NameError # Raised when a variable name is not found.
+-- OSError # Errors such as “file not found” or “disk full” (see Open).
| +-- FileNotFoundError # When a file or directory is requested but doesn't exist.
+-- RuntimeError # Raised by errors that don't fall into other categories.
| +-- RecursionError # Raised when the maximum recursion depth is exceeded.
+-- StopIteration # Raised by next() when run on an empty iterator.
+-- TypeError # Raised when an argument is of wrong type.
+-- ValueError # When an argument is of right type but inappropriate value.
+-- UnicodeError # Raised when encoding/decoding strings to/from bytes fails.
Exits the interpreter by raising SystemExit exception.
importsyssys.exit() # Exits with exit code 0 (success).sys.exit(<el>) # Prints to stderr and exits with 1.sys.exit(<int>) # Exits with passed exit code.
'encoding=None' means that the default encoding is used, which is platform dependent. Best practice is to use 'encoding="utf-8"' whenever possible.
'newline=None' means all different end of line combinations are converted to '\n' on read, while on write all '\n' characters are converted to system's default line separator.
'newline=""' means no conversions take place, but input is still broken into chunks by readline() and readlines() on either '\n', '\r' or '\r\n'.
Modes
'r' - Read (default).
'w' - Write (truncate).
'x' - Write or fail if the file already exists.
'a' - Append.
'w+' - Read and write (truncate).
'r+' - Read and write from the start.
'a+' - Read and write from the end.
't' - Text mode (default).
'b' - Binary mode.
Exceptions
'FileNotFoundError' can be raised when reading with 'r' or 'r+'.
'FileExistsError' can be raised when writing with 'x'.
'IsADirectoryError' and 'PermissionError' can be raised by any.
'OSError' is the parent class of all listed exceptions.
File Object
<file>.seek(0) # Moves to the start of the file.<file>.seek(offset) # Moves 'offset' chars/bytes from the start.<file>.seek(0, 2) # Moves to the end of the file.<bin_file>.seek(±offset, <anchor>) # Anchor: 0 start, 1 current position, 2 end.
<str/bytes>=<file>.read(size=-1) # Reads 'size' chars/bytes or until EOF.<str/bytes>=<file>.readline() # Returns a line or empty string/bytes on EOF.<list>=<file>.readlines() # Returns a list of remaining lines.<str/bytes>=next(<file>) # Returns a line using buffer. Do not mix.
<file>.write(<str/bytes>) # Writes a string or bytes object.<file>.writelines(<collection>) # Writes a coll. of strings or bytes objects.<file>.flush() # Flushes write buffer.
Methods do not add or strip trailing newlines, even writelines().
<str>=getcwd() # Returns the current working directory.<str>=path.join(<path>, ...) # Joins two or more pathname components.<str>=path.abspath(<path>) # Returns absolute path.
<str>=path.basename(<path>) # Returns final component of the path.<str>=path.dirname(<path>) # Returns path without the final component.<tup.>=path.splitext(<path>) # Splits on last period of the final component.
<list>=listdir(path='.') # Returns filenames located at path.<list>=glob('') # Returns paths matching the wildcard pattern.
Using scandir() instead of listdir() can significantly increase the performance of code that also needs file type information.
fromosimportscandir
<iter>=scandir(path='.') # Returns DirEntry objects located at path.<str>=<DirEntry>.path# Returns whole path as a string.<str>=<DirEntry>.name# Returns final component as a string.<file>=open(<DirEntry>) # Opens the file and returns file object.
Path Object
frompathlibimportPath
<Path>=Path(<path> [, ...]) # Accepts strings, Paths and DirEntry objects.<Path>=<path>/<path> [/ ...] # One of the paths must be a Path object.
<Path>=Path() # Returns relative cwd. Also Path('.').<Path>=Path.cwd() # Returns absolute cwd. Also Path().resolve().<Path>=Path.home() # Returns user's home directory.<Path>=Path(__file__).resolve() # Returns script's path if cwd wasn't changed.
<Path>=<Path>.parent# Returns Path without final component.<str>=<Path>.name# Returns final component as a string.<str>=<Path>.stem# Returns final component without extension.<str>=<Path>.suffix# Returns final component's extension.<tup.>=<Path>.parts# Returns all components as strings.
<iter>=<Path>.iterdir() # Returns dir contents as Path objects.<iter>=<Path>.glob('') # Returns Paths matching the wildcard pattern.
<str>=str(<Path>) # Returns path as a string.<file>=open(<Path>) # Opens the file and returns file object.
OS Commands
Files and Directories
Paths can be either strings, Paths or DirEntry objects.
Functions report OS related errors by raising either OSError or one of its subclasses.
importos, shutil
os.chdir(<path>) # Changes the current working directory.os.mkdir(<path>, mode=0o777) # Creates a directory. Mode is in octal.
shutil.copy(from, to) # Copies the file. 'to' can exist or be a dir.shutil.copytree(from, to) # Copies the directory. 'to' must not exist.
os.rename(from, to) # Renames/moves the file or directory.os.replace(from, to) # Same, but overwrites 'to' if it exists.
os.remove(<path>) # Deletes the file.os.rmdir(<path>) # Deletes the empty directory.shutil.rmtree(<path>) # Deletes the directory.
Shell Commands
importos<str>=os.popen('').read()
Sends '1 + 1' to the basic calculator and captures its output:
<reader>=csv.reader(<file>) # Also: `dialect='excel', delimiter=','`.<list>=next(<reader>) # Returns next row as a list of strings.<list>=list(<reader>) # Returns list of remaining rows.
File must be opened with a 'newline=""' argument, or newlines embedded inside quoted fields will not be interpreted correctly!
Server-less database engine that stores each database into a separate file.
Connect
Opens a connection to the database file. Creates a new file if path doesn't exist.
importsqlite3<conn>=sqlite3.connect(<path>) # Also ':memory:'.<conn>.close() # Closes the connection.
Read
Returned values can be of type str, int, float, bytes or None.
<cursor>=<conn>.execute('') # Can raise a subclass of sqlite3.Error.<tuple>=<cursor>.fetchone() # Returns next row. Also next().<list>=<cursor>.fetchall() # Returns remaining rows. Also list().
Write
<conn>.execute('') # Can raise a subclass of sqlite3.Error.<conn>.commit() # Commits all transactions since last commit.
Or:
with<conn>:
<conn>.execute('')
Placeholders
Passed values can be of type str, int, float, bytes, None, bool, datetime.date or datetime.datetme.
<conn>.execute('', <list/tuple>) # Replaces '?'s in query with values.<conn>.execute('', <dict/namedtuple>) # Replaces ':'s with values.<conn>.executemany('', <coll_of_above>) # Runs execute() multiple times.
Example
In this example values are not actually saved because 'conn.commit()' is omitted!
>>>conn=sqlite3.connect('test.db')
>>>conn.execute('CREATE TABLE person (person_id INTEGER PRIMARY KEY, name, height)')
>>>conn.execute('INSERT INTO person VALUES (NULL, ?, ?)', ('Jean-Luc', 187)).lastrowid1>>>conn.execute('SELECT * FROM person').fetchall()
[(1, 'Jean-Luc', 187)]
MySQL
Has a very similar interface, with differences listed below.
# $ pip3 install mysql-connectorfrommysqlimportconnector<conn>=connector.connect(host=<str>, …) # `user=, password=, database=`.<cursor>=<conn>.cursor() # Only cursor has execute method.<cursor>.execute('') # Can raise a subclass of connector.Error.<cursor>.execute('', <list/tuple>) # Replaces '%s's in query with values.<cursor>.execute('', <dict/namedtuple>) # Replaces '%()s's with values.
Bytes
Bytes object is an immutable sequence of single bytes. Mutable version is called bytearray.
<bytes>=b''# Only accepts ASCII characters and \x00-\xff.<int>=<bytes>[<index>] # Returns int in range from 0 to 255.<bytes>=<bytes>[<slice>] # Returns bytes even if it has only one element.<bytes>=<bytes>.join(<coll_of_bytes>) # Joins elements using bytes object as separator.
Encode
<bytes>=bytes(<coll_of_ints>) # Ints must be in range from 0 to 255.<bytes>=bytes(<str>, 'utf-8') # Or: .encode('utf-8')<bytes>=<int>.to_bytes(n_bytes, …) # `byteorder='big/little', signed=False`.<bytes>=bytes.fromhex('') # Hex pairs can be separated by spaces.
Decode
<list>=list(<bytes>) # Returns ints in range from 0 to 255.<str>=str(<bytes>, 'utf-8') # Or: .decode('utf-8')<int>=int.from_bytes(<bytes>, …) # `byteorder='big/little', signed=False`.
'<hex>' =<bytes>.hex() # Returns a string of hexadecimal pairs.
Integer types. Use a capital letter for unsigned type. Minimum and standard sizes are in brackets:
'x' - pad byte
'b' - char (1/1)
'h' - short (2/2)
'i' - int (2/4)
'l' - long (4/4)
'q' - long long (8/8)
Floating point types:
'f' - float (4/4)
'd' - double (8/8)
Array
List that can only hold numbers of a predefined type. Available types and their minimum sizes in bytes are listed above. Sizes and byte order are always determined by the system.
fromarrayimportarray<array>=array('', <collection>) # Array from collection of numbers.<array>=array('', <bytes>) # Array from bytes object.<array>=array('', <array>) # Treats array as a sequence of numbers.<bytes>=bytes(<array>) # Or: .tobytes()
Memory View
A sequence object that points to the memory of another object.
Each element can reference a single or multiple consecutive bytes, depending on format.
Order and number of elements can be changed with slicing.
<mview>=memoryview(<bytes/bytearray/array>) # Immutable if bytes, else mutable.<real>=<mview>[<index>] # Returns an int or a float.<mview>=<mview>[<slice>] # Mview with rearranged elements.<mview>=<mview>.cast('') # Casts memoryview to the new format.<mview>.release() # Releases the object's memory buffer.
Decode
<bin_file>.write(<mview>) # Writes mview to the binary file.<bytes>=bytes(<mview>) # Creates a new bytes object.<bytes>=<bytes>.join(<coll_of_mviews>) # Joins mviews using bytes object as sep.<array>=array('', <mview>) # Treats mview as a sequence of numbers.
<list>=list(<mview>) # Returns list of ints or floats.<str>=str(<mview>, 'utf-8') # Treats mview as a bytes object.<int>=int.from_bytes(<mview>, …) # `byteorder='big/little', signed=False`.
'<hex>' =<mview>.hex() # Treats mview as a bytes object.
Deque
A thread-safe list with efficient appends and pops from either side. Pronounced "deck".
<deque>.appendleft(<el>) # Opposite element is dropped if full.<deque>.extendleft(<collection>) # Collection gets reversed.<el>=<deque>.popleft() # Raises IndexError if empty.<deque>.rotate(n=1) # Rotates elements to the right.
Threading
CPython interpreter can only run a single thread at a time.
That is why using multiple threads won't result in a faster execution, unless at least one of the threads contains an I/O operation.
<Thread>=Thread(target=<function>) # Use `args=` to set arguments.<Thread>.start() # Starts the thread.<bool>=<Thread>.is_alive() # Checks if thread has finished executing.<Thread>.join() # Waits for thread to finish.
Use 'kwargs=' to pass keyword arguments to the function.
Use 'daemon=True', or the program will not be able to exit while the thread is alive.
Lock
<lock>=RLock() # Lock that can only be released by the owner.<lock>.acquire() # Waits for lock to be available.<lock>.release() # Makes lock available again.
Or:
lock=RLock()
withlock:
...
Semaphore, Event, Barrier
<Semaphore>=Semaphore(value=1) # Lock that can be acquired by 'value' threads.<Event>=Event() # Method wait() blocks until set() is called.<Barrier>=Barrier(n_times) # Wait() blocks until it's called n_times.
Thread Pool Executor
Object that manages thread execution.
<Exec>=ThreadPoolExecutor(max_workers=None) # Or: `with ThreadPoolExecutor() as : …`<Exec>.shutdown(wait=True) # Blocks until all threads finish executing.
<iter>=<Exec>.map(<func>, <args_1>, ...) # A multithreaded and non-lazy map().<Futr>=<Exec>.submit(<func>, <arg_1>, ...) # Starts a thread and returns its Future object.<bool>=<Futr>.done() # Checks if the thread has finished executing.<obj>=<Futr>.result() # Waits for thread to finish and returns result.
Queue
A thread-safe FIFO queue. For LIFO queue use LifoQueue.
fromqueueimportQueue<Queue>=Queue(maxsize=0)
<Queue>.put(<el>) # Blocks until queue stops being full.<Queue>.put_nowait(<el>) # Raises queue.Full exception if full.<el>=<Queue>.get() # Blocks until queue stops being empty.<el>=<Queue>.get_nowait() # Raises queue.Empty exception if empty.
Operator
Module of functions that provide the functionality of operators.
<list>=dir() # Names of local variables (incl. functions).<dict>=vars() # Dict of local variables. Also locals().<dict>=globals() # Dict of global variables.
Attributes
<list>=dir(<object>) # Names of object's attributes (incl. methods).<dict>=vars(<object>) # Dict of writable attributes. Also .__dict__.<bool>=hasattr(<object>, '') # Checks if getattr() raises an AttributeError.value=getattr(<object>, '') # Raises AttributeError if attribute is missing.setattr(<object>, '', value) # Only works on objects with __dict__ attribute.delattr(<object>, '') # Equivalent to `del
Parameters
frominspectimportsignature<Sig>=signature(<function>) # Function's Signature object.<dict>=<Sig>.parameters# Dict of function's Parameter objects.<str>=<Param>.name# Parameter's name.<memb>=<Param>.kind# Member of ParameterKind enum.
Metaprograming
Code that generates code.
Type
Type is the root class. If only passed an object it returns its type (class). Otherwise it creates a new class.
New() is a class method that gets called before init(). If it returns an instance of its class, then that instance gets passed to init() as a 'self' argument.
It receives the same arguments as init(), except for the first one that specifies the desired type of the returned instance (MyMetaClass in our case).
Like in our case, new() can also be called directly, usually from a new() method of a child class (def __new__(cls): return super().__new__(cls)).
The only difference between the examples above is that my_meta_class() returns a class of type type, while MyMetaClass() returns a class of type MyMetaClass.
Metaclass Attribute
Right before a class is created it checks if it has the 'metaclass' attribute defined. If not, it recursively checks if any of his parents has it defined and eventually comes to type().
classMyClass(metaclass=MyMetaClass):
b=12345
>>>MyClass.a, MyClass.b
('abcde', 12345)
Type Diagram
type(MyClass) ==MyMetaClass# MyClass is an instance of MyMetaClass.type(MyMetaClass) ==type# MyMetaClass is an instance of type.
Coroutines have a lot in common with threads, but unlike threads, they only give up control when they call another coroutine and they don’t use as much memory.
Coroutine definition starts with 'async' and its call with 'await'.
'asyncio.run()' is the main entry point for asynchronous programs.
Functions wait(), gather() and as_completed() can be used when multiple coroutines need to be started at the same time.
logger.add('debug_{time}.log', colorize=True) # Connects a log file.logger.add('error_{time}.log', level='ERROR') # Another file for errors or higher.logger.<level>('A logging message.')
$ kernprof -lv test.py
Line # Hits Time Per Hit % Time Line Contents
=======================================================
1 @profile
2 def main():
3 1 955.0 955.0 43.7 a = [*range(10000)]
4 1 1231.0 1231.0 56.3 b = {*range(10000)}
$ python3 -m memory_profiler test.py
Line # Mem usage Increment Line Contents
=======================================================
1 37.668 MiB 37.668 MiB @profile
2 def main():
3 38.012 MiB 0.344 MiB a = [*range(10000)]
4 38.477 MiB 0.465 MiB b = {*range(10000)}
Call Graph
Generates a PNG image of a call graph with highlighted bottlenecks:
Array manipulation mini-language. It can run up to one hundred times faster than the equivalent Python code. An even faster alternative that runs on a GPU is called CuPy.
<Image>=Image.new('', (width, height)) # Also: `color=`.<Image>=Image.open(<path>) # Identifies format based on file contents.<Image>=<Image>.convert('') # Converts image to the new mode.<Image>.save(<path>) # Selects format based on the path extension.<Image>.show() # Opens image in default preview app.
<int/tuple>=<Image>.getpixel((x, y)) # Returns a pixel.<Image>.putpixel((x, y), <int/tuple>) # Writes a pixel to the image.<ImagingCore>=<Image>.getdata() # Returns a sequence of pixels.<Image>.putdata(<list/ImagingCore>) # Writes a sequence of pixels.<Image>.paste(<Image>, (x, y)) # Writes an image to the image.
<2d_array> = np.array(<Image_L>) # Creates NumPy array from greyscale image.<3d_array> = np.array(<Image_RGB>) # Creates NumPy array from color image.<Image> = Image.fromarray(<array>) # Creates image from NumPy array of floats.
Modes
'1' - 1-bit pixels, black and white, stored with one pixel per byte.
'L' - 8-bit pixels, greyscale.
'RGB' - 3x8-bit pixels, true color.
'RGBA' - 4x8-bit pixels, true color with transparency mask.
'HSV' - 3x8-bit pixels, Hue, Saturation, Value color space.
<Wave_read>=wave.open('', 'rb') # Opens the WAV file.framerate=<Wave_read>.getframerate() # Number of frames per second.nchannels=<Wave_read>.getnchannels() # Number of samples per frame.sampwidth=<Wave_read>.getsampwidth() # Sample size in bytes.nframes=<Wave_read>.getnframes() # Number of frames.<params>=<Wave_read>.getparams() # Immutable collection of above.<bytes>=<Wave_read>.readframes(nframes) # Returns next 'nframes' frames.
<Wave_write>=wave.open('', 'wb') # Truncates existing file.<Wave_write>.setframerate(<int>) # 44100 for CD, 48000 for video.<Wave_write>.setnchannels(<int>) # 1 for mono, 2 for stereo.<Wave_write>.setsampwidth(<int>) # 2 for CD quality sound.<Wave_write>.setparams(<params>) # Sets all parameters.<Wave_write>.writeframes(<bytes>) # Appends frames to the file.
Bytes object contains a sequence of frames, each consisting of one or more samples.
In a stereo signal, the first sample of a frame belongs to the left channel.
Each sample consists of one or more bytes that, when converted to an integer, indicate the displacement of a speaker membrane at a given moment.
If sample width is one, then the integer should be encoded unsigned.
For all other sizes, the integer should be encoded signed with little-endian byte order.
<Rect>=pg.Rect(x, y, width, height) # Floats get truncated into ints.<int>=<Rect>.x/y/centerx/centery/… # Top, right, bottom, left. Allows assignments.<tup.>=<Rect>.topleft/center/… # Topright, bottomright, bottomleft.<Rect>=<Rect>.move((x, y)) # Use move_ip() to move in place.
<bool>=<Rect>.collidepoint((x, y)) # Checks if rectangle contains a point.<bool>=<Rect>.colliderect(<Rect>) # Checks if two rectangles overlap.<int>=<Rect>.collidelist(<list_of_Rect>) # Returns index of first colliding Rect or -1.<list>=<Rect>.collidelistall(<list_of_Rect>) # Returns indexes of all colliding Rects.
Surface
Object for representing images.
<Surf>=pg.display.set_mode((width, height)) # Returns display surface.<Surf>=pg.Surface((width, height), …) # New RGB surface. Add `pg.SRCALPHA` for RGBA.<Surf>=pg.image.load('') # Loads the image. Format depends on source.<Surf>=<Surf>.subsurface(<Rect>) # Returns a subsurface.
<Surf>.fill(color) # Tuple, Color('#rrggbb[aa]') or Color().<Surf>.set_at((x, y), color) # Updates pixel.<Surf>.blit(<Surf>, (x, y)) # Draws passed surface to the surface.
frompygame.drawimportline, …
line(<Surf>, color, (x1, y1), (x2, y2), width) # Draws a line to the surface.arc(<Surf>, color, <Rect>, from_rad, to_rad) # Also: ellipse(, color, )rect(<Surf>, color, <Rect>) # Also: polygon(, color, points)
Font
<Font>=pg.font.SysFont('', size) # Loads the system font or default if missing.<Font>=pg.font.Font('', size) # Loads the TTF file. Pass None for default.<Surf>=<Font>.render(text, antialias, color) # Background color can be specified at the end.
Sound
<Sound>=pg.mixer.Sound('') # Loads the WAV file.<Sound>.play() # Starts playing the sound.
>>>Series([1, 2], index=['x', 'y'], name='a')
x1y2Name: a, dtype: int64
<Sr>=Series(<list>) # Assigns RangeIndex starting at 0.<Sr>=Series(<dict>) # Takes dictionary's keys for index.<Sr>=Series(<dict/Series>, index=<list>) # Only keeps items with keys specified in index.
<Sr>=<Sr>><==<el/Sr># Returns a Series of bools.<Sr>=<Sr>+-*/<el/Sr># Items with non-matching keys get value NaN.
<Sr>=<Sr>.append(<Sr>) # Or: pd.concat()<Sr>=<Sr>.combine_first(<Sr>) # Adds items that are not yet present.<Sr>.update(<Sr>) # Updates items that are already present.
The way 'aggregate()' and 'transform()' find out whether the passed function accepts an element or the whole Series is by passing it a single value at first and if it raises an error, then they pass it the whole Series.
<DF>=DataFrame(<list_of_rows>) # Rows can be either lists, dicts or series.<DF>=DataFrame(<dict_of_columns>) # Columns can be either lists, dicts or series.
<Sr/DF>=<DF>[column_key/s] # Or: .column_key<DF>=<DF>[row_bools] # Keeps rows as specified by bools.<DF>=<DF>[<DF_of_bools>] # Assigns NaN to False values.
<DF>=<DF>><==<el/Sr/DF># Returns DF of bools. Sr is treated as a row.<DF>=<DF>+-*/<el/Sr/DF># Items with non-matching keys get value NaN.
<DF>=<DF>.set_index(column_key) # Replaces row keys with values from a column.<DF>=<DF>.reset_index() # Moves row keys to a column named index.<DF>=<DF>.filter('', axis=1) # Only keeps columns whose key matches the regex.<DF>=<DF>.melt(id_vars=column_key/s) # Converts DataFrame from wide to long format.
+------------------------+---------------+------------+------------+--------------------------+
| | 'outer' | 'inner' | 'left' | Description |
+------------------------+---------------+------------+------------+--------------------------+
| l.merge(r, on='y', | x y z | x y z | x y z | Joins/merges on column. |
| how=…) | 0 1 2 . | 3 4 5 | 1 2 . | Also accepts left_on and |
| | 1 3 4 5 | | 3 4 5 | right_on parameters. |
| | 2 . 6 7 | | | Uses 'inner' by default. |
+------------------------+---------------+------------+------------+--------------------------+
| l.join(r, lsuffix='l', | x yl yr z | | x yl yr z | Joins/merges on row keys.|
| rsuffix='r', | a 1 2 . . | x yl yr z | 1 2 . . | Uses 'left' by default. |
| how=…) | b 3 4 4 5 | 3 4 4 5 | 3 4 4 5 | If r is a series, it is |
| | c . . 6 7 | | | treated as a column. |
+------------------------+---------------+------------+------------+--------------------------+
| pd.concat([l, r], | x y z | y | | Adds rows at the bottom. |
| axis=0, | a 1 2 . | 2 | | Uses 'outer' by default. |
| join=…) | b 3 4 . | 4 | | A series is treated as a |
| | b . 4 5 | 4 | | column. Use l.append(r) |
| | c . 6 7 | 6 | | to add a row instead. |
+------------------------+---------------+------------+------------+--------------------------+
| pd.concat([l, r], | x y y z | | | Adds columns at the |
| axis=1, | a 1 2 . . | x y y z | | right end. Uses 'outer' |
| join=…) | b 3 4 4 5 | 3 4 4 5 | | by default. A series is |
| | c . . 6 7 | | | treated as a column. |
+------------------------+---------------+------------+------------+--------------------------+
| l.combine_first(r) | x y z | | | Adds missing rows and |
| | a 1 2 . | | | columns. Also updates |
| | b 3 4 5 | | | items that contain NaN. |
| | c . 6 7 | | | R must be a DataFrame. |
+------------------------+---------------+------------+------------+--------------------------+
<GB>=<DF>.groupby(column_key/s) # DF is split into groups based on passed column.<DF>=<GB>.get_group(group_key/s) # Selects a group by value of grouping column.
+-------------+-------------+-------------+-------------+---------------+
| | 'sum' | 'rank' | ['rank'] | {'x': 'rank'} |
+-------------+-------------+-------------+-------------+---------------+
| gb.agg(…) | x y | x y | x y | x |
| | z | a 1 1 | rank rank | a 1 |
| | 3 1 2 | b 1 1 | a 1 1 | b 1 |
| | 6 11 13 | c 2 2 | b 1 1 | c 2 |
| | | | c 2 2 | |
+-------------+-------------+-------------+-------------+---------------+
| gb.trans(…) | x y | x y | | |
| | a 1 2 | a 1 1 | | |
| | b 11 13 | b 1 1 | | |
| | c 11 13 | c 1 1 | | |
+-------------+-------------+-------------+-------------+---------------+
Thanks for the nice cheatsheet 😃
So I saw you wanted help for Better mobile experience and I made some changes:
added viewport meta tag for the fonts to scale properly
moved the *.js files to the end of the html document, to guarante that the document is ready before they get executed
changed the way links are inserted in the headings, so that there is no need for absolute positions and negativ margins and a min-width for the html document
changed the url for the font so it also works locally
h1 width and silly walks banner now scale with the screen width
Sorry about the barrage of pull requests. I just found this Cheat Sheet, and because I think it's an awesome resource, I have this strong urge to suggest improvements.
This one combines three additions to the Regex section to keep the PR volume low. I hope that's okay; feel free to take or leave what you think is appropriate. I'll also happily split this up if you prefer. (Also, this is it from me for today. :grin:)
Warn about the default matching of unicode character classes. This is a serious source of bugs.
Show that patterns can be pre-compiled and reused for efficiency gains.
Mention help(re), because it's a very good resource.
Hi, I just wanted to add a table of contents to the page, to ease navigation and discovery.
I didn't test the static website part however (style for the new table of contents).
Thank you for this great cheatsheet!
I went to try this.. and found that the from itertools import * makes the call to itertools invisible, and totally not obvious to a newbie like me! I'm ok with just 'import itertools' and then being explicit. Huh. Ok, I just referenced the Zen of Python.. must have reached newbie level 2!
Could not get first example to show the result as I needed the right incantation of list(itertools.combinations('abc', 2)), so I added it as an example.
May be worth it to tweak things just a little.. and only keep something like my version of line 910 and ignore the rest :-)
out = []
for i in range(10):
for j in range(10):
out.append(i+j)
The loop variables i and j should be swapped in either the first or the second part. Maybe one of the 10s could even be changed into another value to make it clearer? For instance:
out = [i+j for j in range(5) for i in range(10)]
Is the same as:
out = []
for i in range(10):
for j in range(5):
out.append(i+j)
In the Mario game code snippet the run function currently passes None into 'pressed' dict. disabling controls. Revised run function below fixes. Per PG documentation, get_pressed() isn't the best way to get text entry from user: https://www.pygame.org/docs/ref/key.html#pygame.key.get_pressed
changed confising phrases for any and all functions. Fixed code explaining all function to support all values instead of just iterable elements in the collection.
os.makedirs is a great way to create directories and sub directories recursively. Existing directories can also be ignored by setting the exist_ok argument to True.
In python 3.8 there is a more elegant way to calculate the product of the elements.
Instead of
product_of_elems = functools.reduce(lambda out, el: out * el, <collection>)
it can be done like this
product_of_elems = math.prod(<collection>)
Hello,
I think adding a licence file would be useful as the actual licence of the project is a bit ambiguous (it is by default not possible legally to use any of the code in the cheatsheet).
Is it on purpose ?
Read thru the source, and in the process markdown linted and reformated the source for readability.
I don't know how/if I can upload the git commit, so I attached source file here.
Feel free to use any of my suggested changes to the source.
Thanks
README.md
I love the coroutine game! To me, it's a lot more fun when the numbers "chase" the player, which is enabled in this PR.
Also, running the example as-is generates a deprecation warning for me, so I changed asyncio.wait(coro) to generating tasks as recommended here: https://stackoverflow.com/questions/66029772/alternative-to-asyncio-wait
Package pyVHR (short for Python framework for Virtual Heart Rate) is a comprehensive framework for studying methods of pulse rate estimation relying on remote photoplethysmography (rPPG)
FollowSpot is a comprehensive audition tracking fullstack web application for entertainment industry professionals. This app allows users to store information/media for all of their auditions while also compiling data and displaying statistics to help track progress.
PyDy, short for Python Dynamics, is a tool kit written in the Python programming language that utilizes an array of scientific programs to enable the study of multibody dynamics. The goal is to have a modular framework and eventually a physics abstraction layer which utilizes a variety of backends that can provide the user with their desired workflow
Run python scripts and pass data between multiple python and node processes using this npm module. process-communication has a event based architecture for interacting with python data and errors inside nodejs.