Comprehensive Python Cheatsheet
===============================
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Main
----
```python
if __name__ == '__main__':
main()
```
List
----
```python
<list> = <list>[from_inclusive : to_exclusive : step_size]
<list>.append(<el>)
<list>.extend(<list>)
<list> += [<el>]
<list> += <list>
```
```python
<list>.sort()
<list>.reverse()
<list> = sorted(<list>)
<iter> = reversed(<list>)
```
```python
sum_of_elements = sum(<list>)
elementwise_sum = [sum(pair) for pair in zip(list_a, list_b)]
sorted_by_second = sorted(<list>, key=lambda el: el[1])
sorted_by_both = sorted(<list>, key=lambda el: (el[1], el[0]))
flattened_list = list(itertools.chain.from_iterable(<list>))
list_of_chars = list(<str>)
product_of_elems = functools.reduce(lambda out, x: out * x, <list>)
no_duplicates = list(dict.fromkeys(<list>))
```
```python
index = <list>.index(<el>) # Returns first index of item.
<list>.insert(index, <el>) # Inserts item at index and moves the rest to the right.
<el> = <list>.pop([index]) # Removes and returns item at index or from the end.
<list>.remove(<el>) # Removes first occurrence of item.
<list>.clear() # Removes all items.
```
Dictionary
----------
```python
<view> = <dict>.keys()
<view> = <dict>.values()
<view> = <dict>.items()
```
```python
value = <dict>.get(key, default) # Returns default if key does not exist.
value = <dict>.setdefault(key, default) # Same, but also adds default to dict.
<dict> = collections.defaultdict(<type>) # Creates a dictionary with default value of type.
<dict> = collections.defaultdict(lambda: 1) # Creates a dictionary with default value 1.
```
```python
<dict>.update(<dict>)
<dict> = dict(<list>) # Initiates a dict from list of key-value pairs.
<dict> = dict(zip(keys, values)) # Initiates a dict from two lists.
<dict> = dict.fromkeys(keys [, value]) # Initiates a dict from list of keys.
{k: v for k, v in <dict>.items() if k in keys} # Filters a dict by keys.
```
### Counter
```python
>>> from collections import Counter
>>> colors = ['blue', 'red', 'blue', 'yellow', 'blue', 'red']
>>> Counter(colors)
Counter({'blue': 3, 'red': 2, 'yellow': 1})
>>> <counter>.most_common()[0][0]
'blue'
```
Set
---
```python
<set> = set()
<set>.add(<el>)
<set>.update(<set>)
<set>.clear()
```
```python
<set> = <set>.union(<set>) # Or: <set> | <set>
<set> = <set>.intersection(<set>) # Or: <set> & <set>
<set> = <set>.difference(<set>) # Or: <set> - <set>
<set> = <set>.symmetric_difference(<set>) # Or: <set> ^ <set>
<bool> = <set>.issubset(<set>) # Or: <set> < <set>
<bool> = <set>.issuperset(<set>) # Or: <set> > <set>
```
### Frozenset
#### Is hashable and can be used as a key in dictionary.
```python
<frozenset> = frozenset(<collection>)
```
Range
-----
```python
range(to_exclusive)
range(from_inclusive, to_exclusive)
range(from_inclusive, to_exclusive, step_size)
range(from_inclusive, to_exclusive, -step_size)
```
```python
from_inclusive = <range>.start
to_exclusive = <range>.stop
```
Enumerate
---------
```python
for i, <el> in enumerate(<collection> [, i_start]):
...
```
Named Tuple
-----------
```python
>>> Point = collections.namedtuple('Point', 'x y')
>>> p = Point(1, y=2)
Point(x=1, y=2)
>>> p[0]
1
>>> p.x
1
>>> getattr(p, 'y')
2
>>> p._fields # Or: Point._fields
('x', 'y')
```
Iterator
--------
#### Skips first element:
```python
next(<iter>)
for element in <iter>:
...
```
#### Reads input until it reaches an empty line:
```python
for line in iter(input, ''):
...
```
#### Same, but prints a message every time:
```python
from functools import partial
for line in iter(partial(input, 'Please enter value: '), ''):
...
```
Generator
---------
**Convenient way to implement the iterator protocol.**
```python
def step(start, step):
while True:
yield start
start += step
```
```python
>>> stepper = step(10, 2)
>>> next(stepper), next(stepper), next(stepper)
(10, 12, 14)
```
Type
----
```python
<type> = type(<el>) # <class 'int'> / <class 'str'> / ...
```
```python
from numbers import Number, Integral, Real, Rational, Complex
is_number = isinstance(<el>, Number)
is_function = callable(<el>)
```
String
------
```python
<str> = <str>.strip() # Strips all whitespace characters.
<str> = <str>.strip('<chars>') # Strips all passed characters.
```
```python
<list> = <str>.split() # Splits on any whitespace character.
<list> = <str>.split(sep=None, maxsplit=-1) # Splits on 'sep' at most 'maxsplit' times.
<str> = <str>.join(<list>) # Joins elements using string as separator.
```
```python
<str> = <str>.replace(old_str, new_str)
<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>.index(<sub_str>) # Returns first index of a substring.
<bool> = <str>.isnumeric() # True if str contains only numeric characters.
<list> = textwrap.wrap(<str>, width) # Nicely breaks string into lines.
```
### Char
```python
<str> = chr(<int>) # Converts int to unicode char.
<int> = ord(<str>) # Converts unicode char to int.
```
```python
>>> ord('0'), ord('9')
(48, 57)
>>> ord('A'), ord('Z')
(65, 90)
>>> ord('a'), ord('z')
(97, 122)
```
### Print
```python
print(<el_1> [, <el_2>, end='', sep='', file=<file>]) # Use 'file=sys.stderr' for errors.
```
```python
>>> from pprint import pprint
>>> pprint(locals())
{'__doc__': None,
'__name__': '__main__',
'__package__': None, ...}
```
Regex
-----
```python
import re
<str> = re.sub(<regex>, new, text, count=0) # Substitutes all occurrences.
<list> = re.findall(<regex>, text)
<list> = re.split(<regex>, text, maxsplit=0) # Use brackets in regex to keep the matches.
<Match> = re.search(<regex>, text) # Searches for first occurrence of pattern.
<Match> = re.match(<regex>, text) # Searches only at the beginning of the text.
<Match_iter> = re.finditer(<regex>, text) # Searches for all occurrences of pattern.
```
* **Parameter `'flags=re.IGNORECASE'` can be used with all functions.**
* **Parameter `'flags=re.DOTALL'` makes dot also accept newline.**
* **Use `r'\1'` or `'\\\\1'` for backreference.**
* **Use `'?'` to make operators non-greedy.**
### Match Object
```python
<str> = <Match>.group() # Whole match.
<str> = <Match>.group(1) # Part in first bracket.
<int> = <Match>.start() # Start index of a match.
<int> = <Match>.end() # Exclusive end index of a match.
```
### Special Sequences
**Use capital letter for negation.**
```python
'\d' == '[0-9]' # Digit
'\s' == '[ \t\n\r\f\v]' # Whitespace
'\w' == '[a-zA-Z0-9_]' # Alphanumeric
```
Format
------
```python
<str> = f'{<el_1>}, {<el_2>}'
<str> = '{}, {}'.format(<el_1>, <el_2>)
```
```python
>>> Person = namedtuple('Person', 'name height')
>>> person = Person('Jean-Luc', 187)
>>> f'{person.height:10}'
' 187'
>>> '{p.height:10}'.format(p=person)
' 187'
```
### General Options
```python
{<el>:<10} # '<el> '
{<el>:>10} # ' <el>'
{<el>:^10} # ' <el> '
{<el>:->10} # '------<el>'
{<el>:>0} # '<el>'
```
### String Options
**"!r" uses object's repr() method, instead of format(), to get a string:**
```python
{'abcde'!r} # "'abcde'"
```
```python
{'abcde':.3} # 'abc'
{'abcde':10.3} # 'abc '
```
### Number Options
```python
{1.23456:.3f} # '1.235'
{1.23456:10.3f} # ' 1.235'
```
```python
{123456:10,} # ' 123,456'
{123456:10_} # ' 123_456'
{123456:+10} # ' +123456'
{-123456:=10} # '- 123456'
{123456: } # ' 123456'
{-123456: } # '-123456'
```
```python
{65:c} # 'A'
{3:08b} # '00000011' -> Binary with leading zeros.
{3:0<8b} # '11000000' -> Binary with trailing zeros.
```
#### Float presentation types:
* `'f'` - Fixed point: `.<precision>f`
* `'%'` - Percent: `.<precision>%`
* `'e'` - Exponent
#### Integer presentation types:
* `'c'` - Character
* `'b'` - Binary
* `'x'` - Hex
* `'X'` - HEX
Numbers
-------
### Basic Functions
```python
pow(x, y) # Or: x ** y
abs(<num>)
round(<num> [, ndigits])
```
### Constants
```python
from math import e, pi
```
### Trigonometry
```python
from math import cos, acos, sin, asin, tan, atan, degrees, radians
```
### Logarithm
```python
from math import log, log10, log2
log(x [, base]) # Base e, if not specified.
log10(x) # Base 10.
log2(x) # Base 2.
```
### Infinity, nan
```python
from math import inf, nan, isinf, isnan
```
#### Or:
```python
float('inf'), float('nan')
```
### Random
```python
from random import random, randint, choice, shuffle
<float> = random()
<int> = randint(from_inclusive, to_inclusive)
<el> = choice(<list>)
shuffle(<list>)
```
Datetime
--------
```python
from datetime import datetime, strptime
now = datetime.now()
now.month # 3
now.strftime('%Y%m%d') # '20180315'
now.strftime('%Y%m%d%H%M%S') # '20180315002834'
<datetime> = strptime('2015-05-12 00:39', '%Y-%m-%d %H:%M')
```
Arguments
---------
**"*" is the splat operator, that takes a list as input, and expands it into actual positional arguments in the function call.**
```python
args = (1, 2)
kwargs = {'x': 3, 'y': 4, 'z': 5}
func(*args, **kwargs)
```
#### Is the same as:
```python
func(1, 2, x=3, y=4, z=5)
```
#### Splat operator can also be used in function declarations:
```python
def add(*a):
return sum(a)
```
```python
>>> add(1, 2, 3)
6
```
#### And in few other places:
```python
>>> a = (1, 2, 3)
>>> [*a]
[1, 2, 3]
```
```python
>>> head, *body, tail = [1, 2, 3, 4]
>>> body
[2, 3]
```
Inline
------
### Lambda
```python
lambda: <return_value>
lambda <argument_1>, <argument_2>: <return_value>
```
### Comprehension
```python
<list> = [i+1 for i in range(10)] # [1, 2, ..., 10]
<set> = {i for i in range(10) if i > 5} # {6, 7, ..., 9}
<dict> = {i: i*2 for i in range(10)} # {0: 0, 1: 2, ..., 9: 18}
<iter> = (x+5 for x in range(10)) # (5, 6, ..., 14)
```
```python
out = [i+j for i in range(10) for j in range(10)]
```
#### Is the same as:
```python
out = []
for i in range(10):
for j in range(10):
out.append(i+j)
```
### Map, Filter, Reduce
```python
from functools import reduce
<iter> = map(lambda x: x + 1, range(10)) # (1, 2, ..., 10)
<iter> = filter(lambda x: x > 5, range(10)) # (6, 7, ..., 9)
<any_type> = reduce(lambda out, x: out + x, range(10)) # 45
```
### Any, All
```python
<bool> = any(el[1] for el in <collection>)
```
### If - Else
```python
<expression_if_true> if <condition> else <expression_if_false>
```
```python
>>> [a if a else 'zero' for a in (0, 1, 0, 3)]
['zero', 1, 'zero', 3]
```
### Namedtuple, Enum, Class
```python
from collections import namedtuple
Point = namedtuple('Point', 'x y')
from enum import Enum
Direction = Enum('Direction', 'n e s w')
Cutlery = Enum('Cutlery', {'knife': 1, 'fork': 2, 'spoon': 3})
# Warning: Objects will share the objects that are initialized in the dictionary!
Creature = type('Creature', (), {'position': Point(0, 0), 'direction': Direction.n})
creature = Creature()
```
Closure
-------
```python
def get_multiplier(a):
def out(b):
return a * b
return out
```
```python
>>> multiply_by_3 = get_multiplier(3)
>>> multiply_by_3(10)
30
```
#### Or:
```python
from functools import partial
<function> = partial(<function>, <argument_1> [, <argument_2>, ...])
```
```python
>>> multiply_by_3 = partial(operator.mul, 3)
>>> multiply_by_3(10)
30
```
Decorator
---------
```python
@closure_name
def function_that_gets_passed_to_closure():
...
```
#### Debugger example:
```python
from functools import wraps
def debug(func):
@wraps(func) # Needed for metadata copying (func name, ...).
def out(*args, **kwargs):
print(func.__name__)
return func(*args, **kwargs)
return out
@debug
def add(x, y):
return x + y
```
Class
-----
```python
class <name>:
def __init__(self, a):
self.a = a
def __str__(self):
return str(self.a)
def __repr__(self):
return str({'a': self.a}) # Or: return f'{self.__dict__}'
@classmethod
def get_class_name(cls):
return cls.__name__
```
### Constructor Overloading
```python
class <name>:
def __init__(self, a=None):
self.a = a
```
### Inheritance
```python
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
class Employee(Person):
def __init__(self, name, age, staff_num):
super().__init__(name, age)
self.staff_num = staff_num
```
### Copy
```python
from copy import copy, deepcopy
<object> = copy(<object>)
<object> = deepcopy(<object>)
```
Enum
----
```python
from enum import Enum, auto
class <enum_name>(Enum):
<member_name_1> = <value_1>
<member_name_2> = <value_2_a>, <value_2_b>
<member_name_3> = auto() # Can be used for automatic indexing.
...
@classmethod
def get_names(cls):
return [a.name for a in cls.__members__.values()]
```
```python
<member> = <enum>.<member_name>
<member> = <enum>['<member_name>']
<member> = <enum>(<value>)
<name> = <member>.name
<value> = <member>.value
```
```python
list_of_members = list(<enum>)
member_names = [a.name for a in <enum>]
member_values = [a.value for a in <enum>]
random_member = random.choice(list(<enum>))
```
### Inline
```python
Cutlery = Enum('Cutlery', ['knife', 'fork', 'spoon'])
Cutlery = Enum('Cutlery', 'knife fork spoon')
Cutlery = Enum('Cutlery', {'knife': 1, 'fork': 2, 'spoon': 3})
# Functions can not be values, so they must be enclosed in tuple:
LogicOp = Enum('LogicOp', {'AND': (lambda l, r: l and r, ),
'OR' : (lambda l, r: l or r, )})
# But 'list(<enum>)' will only work if there is another value in the tuple:
LogicOp = Enum('LogicOp', {'AND': (auto(), lambda l, r: l and r),
'OR' : (auto(), lambda l, r: l or r)})
```
System
------
### Arguments
```python
import sys
script_name = sys.argv[0]
arguments = sys.argv[1:]
```
### Read File
```python
def read_file(filename):
with open(filename, encoding='utf-8') as file:
return file.readlines()
```
### Write to File
```python
def write_to_file(filename, text):
with open(filename, 'w', encoding='utf-8') as file:
file.write(text)
```
### Path
```python
from os import path, listdir
<bool> = path.exists(<path>)
<bool> = path.isfile(<path>)
<bool> = path.isdir(<path>)
<list> = listdir(<path>)
```
```python
>>> from glob import glob
>>> glob('../*.gif')
['1.gif', 'card.gif']
```
### Execute Command
```python
import os
<str> = os.popen(<command>).read()
```
#### Or:
```python
>>> import subprocess
>>> a = subprocess.run(['ls', '-a'], stdout=subprocess.PIPE)
>>> a.stdout
b'.\n..\nfile1.txt\nfile2.txt\n'
>>> a.returncode
0
```
### Input
```python
filename = input('Enter a file name: ')
```
#### Prints lines until EOF:
```python
while True:
try:
print(input())
except EOFError:
break
```
### Recursion Limit
```python
>>> sys.getrecursionlimit()
1000
>>> sys.setrecursionlimit(10000)
```
JSON
----
```python
import json
```
### Serialization
```python
<str> = json.dumps(<object>, ensure_ascii=True, indent=None)
<dict> = json.loads(<str>)
```
#### To preserve order:
```python
from collections import OrderedDict
<dict> = json.loads(<str>, object_pairs_hook=OrderedDict)
```
### Read File
```python
def read_json_file(filename):
with open(filename, encoding='utf-8') as file:
return json.load(file)
```
### Write to File
```python
def write_to_json_file(filename, an_object):
with open(filename, 'w', encoding='utf-8') as file:
json.dump(an_object, file, ensure_ascii=False, indent=2)
```
SQLite
------
```python
import sqlite3
db = sqlite3.connect(<filename>)
```
### Read
```python
cursor = db.execute(<query>)
if cursor:
<tuple> = cursor.fetchone() # First row.
<list> = cursor.fetchall() # Remaining rows.
db.close()
```
### Write
```python
db.execute(<query>)
db.commit()
```
Pickle
------
```python
>>> import pickle
>>> favorite_color = {'lion': 'yellow', 'kitty': 'red'}
>>> pickle.dump(favorite_color, open('data.p', 'wb'))
>>> pickle.load(open('data.p', 'rb'))
{'lion': 'yellow', 'kitty': 'red'}
```
Exceptions
----------
```python
while True:
try:
x = int(input('Please enter a number: '))
except ValueError:
print('Oops! That was no valid number. Try again...')
else:
print('Thank you.')
break
```
#### Raising exception:
```python
raise ValueError('A very specific message!')
```
### Finally
```python
>>> try:
... raise KeyboardInterrupt
... finally:
... print('Goodbye, world!')
Goodbye, world!
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
KeyboardInterrupt
```
Bytes
-----
**Bytes objects are immutable sequences of single bytes.**
### Encode
```python
<Bytes> = b'<str>'
<Bytes> = <str>.encode(encoding='utf-8')
<Bytes> = <int>.to_bytes(<length>, byteorder='big|little', signed=False)
<Bytes> = bytes.fromhex(<hex>)
```
### Decode
```python
<str> = <Bytes>.decode('utf-8')
<int> = int.from_bytes(<Bytes>, byteorder='big|little', signed=False)
<hex> = <Bytes>.hex()
```
### Read Bytes from File
```python
def read_bytes(filename):
with open(filename, 'rb') as file:
return file.read()
```
### Write Bytes to File
```python
def write_bytes(filename, bytes):
with open(filename, 'wb') as file:
file.write(bytes)
```
```python
<Bytes> = b''.join(<list_of_Bytes>)
```
Struct
------
**This module performs conversions between Python values and C struct represented as Python Bytes object.**
```python
from struct import pack, unpack
<Bytes> = pack('<format>', <value_1> [, <value_2>, ...])
<tuple> = unpack('<format>', <Bytes>)
```
### Example
```python
>>> pack('hhl', 1, 2, 3)
b'\x00\x01\x00\x02\x00\x00\x00\x03'
>>> unpack('hhl', b'\x00\x01\x00\x02\x00\x00\x00\x03')
(1, 2, 3)
>>> calcsize('hhl')
8
```
### Format
**Use capital leters for unsigned type.**
* `'x'` - pad byte
* `'c'` - char
* `'h'` - short
* `'i'` - int
* `'l'` - long
* `'q'` - long long
* `'f'` - float
* `'d'` - double
Hashlib
-------
```python
>>> hashlib.md5(<str>.encode()).hexdigest()
'33d0eba106da4d3ebca17fcd3f4c3d77'
```
Threading
---------
```python
from threading import Thread, RLock
```
### Thread
```python
thread = Thread(target=<function>, args=(<first_arg>, ))
thread.start()
...
thread.join()
```
### Lock
```python
lock = Rlock()
lock.acquire()
...
lock.release()
```
Itertools
---------
**Every function returns an iterator and can accept any collection and/or iterator. If you want to print the iterator, you need to pass it to the list() function.**
```python
from itertools import *
```
### Combinatoric iterators
```python
>>> combinations('abc', 2)
[('a', 'b'), ('a', 'c'), ('b', 'c')]
>>> combinations_with_replacement('abc', 2)
[('a', 'a'), ('a', 'b'), ('a', 'c'),
('b', 'b'), ('b', 'c'),
('c', 'c')]
>>> permutations('abc', 2)
[('a', 'b'), ('a', 'c'),
('b', 'a'), ('b', 'c'),
('c', 'a'), ('c', 'b')]
>>> product('ab', [1, 2])
[('a', 1), ('a', 2),
('b', 1), ('b', 2)]
>>> product([0, 1], repeat=3)
[(0, 0, 0), (0, 0, 1), (0, 1, 0), (0, 1, 1),
(1, 0, 0), (1, 0, 1), (1, 1, 0), (1, 1, 1)]
```
### Infinite iterators
```python
>>> i = count(5, 2)
>>> next(i), next(i), next(i)
(5, 7, 9)
>>> a = cycle('abc')
>>> [next(a) for _ in range(10)]
['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c', 'a']
>>> repeat(10, 3)
[10, 10, 10]
```
### Iterators
```python
>>> chain([1, 2], range(3, 5))
[1, 2, 3, 4]
>>> compress('abc', [True, 0, 1])
['a', 'c']
>>> islice([1, 2, 3], 1, None) # islice(<seq>, from_inclusive, to_exclusive)
[2, 3]
>>> people = [{'id': 1, 'name': 'bob'},
{'id': 2, 'name': 'bob'},
{'id': 3, 'name': 'peter'}]
>>> {name: list(ppp) for name, ppp in groupby(people, key=lambda p: p['name'])}
{'bob': [{'id': 1, 'name': 'bob'},
{'id': 2, 'name': 'bob'}],
'peter': [{'id': 3, 'name': 'peter'}]}
```
Introspection and Metaprograming
--------------------------------
**Inspecting code at runtime and code that generates code. You can:**
* **Look at the attributes**
* **Set new attributes**
* **Create functions dynamically**
* **Traverse the parent classes**
* **Change values in the class**
### Variables
```python
<list> = dir() # In-scope variables.
<dict> = locals() # Local variables.
<dict> = globals() # Global variables.
```
### Attributes
```python
>>> class Z:
... def __init__(self):
... self.a = 'abcde'
... self.b = 12345
>>> z = Z()
```
```python
>>> vars(z)
{'a': 'abcde', 'b': 12345}
>>> getattr(z, 'a')
'abcde'
>>> hasattr(z, 'c')
False
>>> setattr(z, 'c', 10)
```
### Parameters
#### Getting the number of parameters of a function:
```python
from inspect import signature
sig = signature(<function>)
no_of_params = len(sig.parameters)
```
### Type
**Type is the root class. If only passed the object it returns it's type. Otherwise it creates a new class (and not the instance!):**
```python
type(<class_name>, <parents_tuple>, <attributes_dict>)
```
```python
>>> Z = type('Z', (), {'a': 'abcde', 'b': 12345})
>>> z = Z()
```
### MetaClass
#### Class that creates class:
```python
def my_meta_class(name, parents, attrs):
...
return type(name, parents, attrs)
```
#### Or:
```python
class MyMetaClass(type):
def __new__(klass, name, parents, attrs):
...
return type.__new__(klass, name, parents, attrs)
```
### Metaclass Attribute
**When class is created it checks if it has metaclass defined. If not, it recursively checks if any of his parents has it defined, and eventually comes to type:**
```python
class BlaBla:
__metaclass__ = Bla
```
Operator
--------
```python
from operator import add, sub, mul, truediv, floordiv, mod, pow, neg, abs, \
eq, ne, lt, le, gt, ge, \
not_, and_, or_, \
itemgetter
```
```python
product_of_elems = functools.reduce(mul, <list>)
sorted_by_second = sorted(<list>, key=itemgetter(1))
sorted_by_both = sorted(<list>, key=itemgetter(0, 1))
```
Eval
----
### Basic
```python
>>> from ast import literal_eval
>>> literal_eval('1 + 2')
3
>>> literal_eval('[1, 2, 3]')
[1, 2, 3]
```
### Using Abstract Syntax Trees
```python
import ast
from ast import Num, BinOp, UnaryOp
import operator as op
legal_operators = {ast.Add: op.add,
ast.Sub: op.sub,
ast.Mult: op.mul,
ast.Div: op.truediv,
ast.Pow: op.pow,
ast.BitXor: op.xor,
ast.USub: op.neg}
def evaluate(expression):
root = ast.parse(expression, mode='eval')
return eval_node(root.body)
def eval_node(node):
node_type = type(node)
if node_type == Num:
return node.n
if node_type not in [BinOp, UnaryOp]:
raise TypeError(node)
operator_type = type(node.op)
if operator_type not in legal_operators:
raise TypeError(f'Illegal operator {node.op}')
operator = legal_operators[operator_type]
if node_type == BinOp:
left, right = eval_node(node.left), eval_node(node.right)
return operator(left, right)
elif node_type == UnaryOp:
operand = eval_node(node.operand)
return operator(operand)
```
```python
>>> evaluate('2^6')
4
>>> evaluate('2**6')
64
>>> evaluate('1 + 2*3**(4^5) / (6 + -7)')
-5.0
```
Coroutine
---------
* **Similar to Generator, but Generator pulls data through the pipe with iteration, while Coroutine pushes data into the pipeline with send().**
* **Coroutines provide more powerful data routing possibilities than iterators.**
* **If you built a collection of simple data processing components, you can glue them together into complex arrangements of pipes, branches, merging, etc.**
### Helper Decorator
* **All coroutines must be "primed" by first calling next().**
* **Remembering to call next() is easy to forget.**
* **Solved by wrapping coroutines with a decorator:**
```python
def coroutine(func):
def out(*args, **kwargs):
cr = func(*args, **kwargs)
next(cr)
return cr
return out
```
### Pipeline Example
```python
def reader(target):
for i in range(10):
target.send(i)
target.close()
@coroutine
def adder(target):
while True:
item = (yield)
target.send(item + 100)
@coroutine
def printer():
while True:
item = (yield)
print(item)
reader(adder(printer()))
```
<br><br>
Libraries
=========
Plot
----
```python
# $ pip3 install matplotlib
from matplotlib import pyplot
pyplot.plot(<data_1> [, <data_2>, ...])
pyplot.show()
pyplot.savefig(<filename>, transparent=True)
```
Progress Bar
------------
```python
# $ pip3 install tqdm
from tqdm import tqdm
from time import sleep
for i in tqdm(range(100)):
sleep(0.02)
for i in tqdm([1, 2, 3]):
sleep(0.2)
```
Table
-----
#### Prints CSV file as ASCII table:
```python
# $ pip3 install tabulate
from csv import reader
from tabulate import tabulate
with open(<filename>, newline='') as csv_file:
reader = reader(csv_file, delimiter=';')
headers = [a.title() for a in next(reader)]
print(tabulate(reader, headers))
```
Curses
------
```python
# $ pip3 install curses
from curses import wrapper
def main():
wrapper(draw)
def draw(screen):
screen.clear()
screen.addstr(0, 0, 'Press ESC to quit.')
while screen.getch() != 27:
pass
def get_border(screen):
from collections import namedtuple
P = namedtuple('P', 'x y')
height, width = screen.getmaxyx()
return P(width - 1, height - 1)
```
Image
-----
#### Creates PNG image of greyscale gradient:
```python
# $ pip3 install pillow
from PIL import Image
width, height = 100, 100
img = Image.new('L', (width, height), 'white')
img.putdata([255*a/(width*height) for a in range(width*height)])
img.save('out.png')
```
### 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.
Audio
-----
#### Saves a list of floats with values between 0 and 1 to a WAV file:
```python
import wave, struct
frames = [struct.pack('h', int((a-0.5)*60000)) for a in <list>]
wf = wave.open(<filename>, 'wb')
wf.setnchannels(1)
wf.setsampwidth(4)
wf.setframerate(44100)
wf.writeframes(b''.join(frames))
wf.close()
```
Url
---
```python
from urllib.parse import quote, quote_plus, unquote, unquote_plus
```
### Encode
```python
>>> quote("Can't be in URL!")
'Can%27t%20be%20in%20URL%21'
>>> quote_plus("Can't be in URL!")
'Can%27t+be+in+URL%21'
```
### Decode
```python
>>> unquote('Can%27t+be+in+URL%21')
"Can't+be+in+URL!"'
>>> unquote_plus('Can%27t+be+in+URL%21')
"Can't be in URL!"
```
Scraping
--------
```python
# $ pip3 install requests beautifulsoup4
>>> import requests
>>> from bs4 import BeautifulSoup
>>> page = requests.get('https://en.wikipedia.org/wiki/Python_(programming_language)')
>>> document = BeautifulSoup(page.text, 'html.parser')
>>> table = document.find('table', class_='infobox vevent')
>>> rows = table.find_all('tr')
>>> website = rows[11].find('a')['href']
'https://www.python.org/'
>>> latest_v = rows[6].find('div').text.split()[0]
'3.7.2'
```
Web
---
```python
# $ pip3 install bottle
import bottle
from urllib.parse import unquote
```
### Run
```python
bottle.run(host='localhost', port=8080)
bottle.run(host='0.0.0.0', port=80, server='cherrypy')
```
### Static Request
```python
@route('/img/<image>')
def send_image(image):
return static_file(image, 'images/', mimetype='image/png')
```
### Dynamic Request
```python
@route('/<sport>')
def send_page(sport):
sport = unquote(sport).lower()
page = read_file(sport)
return template(page)
```
### REST Request
```python
@post('/odds/<sport>')
def odds_handler(sport):
team = bottle.request.forms.get('team')
team = unquote(team).lower()
db = sqlite3.connect(<db_path>)
home_odds, away_odds = get_odds(db, sport, team)
db.close()
response.headers['Content-Type'] = 'application/json'
response.headers['Cache-Control'] = 'no-cache'
return json.dumps([home_odds, away_odds])
```
Profile
-------
### Basic
```python
from time import time
start_time = time()
...
duration = time() - start_time
```
### Timing a Snippet
```python
from timeit import timeit
timeit('"-".join(str(a) for a in range(100))',
number=10000, globals=globals())
```
### Call Graph
#### Generates a PNG image of call graph with highlighted bottlenecks:
```python
# $ pip3 install pycallgraph
from pycallgraph import output, PyCallGraph
from datetime import datetime
graph = output.GraphvizOutput()
time_str = datetime.now().strftime('%Y%m%d%H%M%S')
graph.output_file = f'profile-{time_str}.png'
with PyCallGraph(output=graph):
<code_to_be_profiled>
```
### Profiling Decorators
#### Prints runtime of a decorated function:
```python
from timeit import default_timer
from datetime import timedelta
def time_me(func):
def out(*args, **kwargs):
start = default_timer()
result = func(*args, **kwargs)
delta = timedelta(seconds=(default_timer() - start))
print(f'Function {func.__name__} finished in {delta}')
return result
return out
```
#### Saves run call profile of a decorated function to file:
```python
from cProfile import Profile
from pstats import Stats
def profile_me(func):
def out(*args, **kwargs):
profile = Profile()
result = profile.runcall(func, *args, **kwargs)
filename = f'profile_{func.__name__}.txt'
with open(filename, 'w') as stream:
stats = Stats(profile, stream=stream)
stats.strip_dirs().sort_stats('tottime')
stats.print_stats(20)
print(f"Profile saved as '{filename}'")
return result
return out
```
#### Prints arguments and output of a decorated function:
```python
def trace_me(func):
def out(*args, **kwargs):
result = func(*args, **kwargs)
arg_list = [repr(x) for x in args]
arg_list += [f'{k}={v!r}' for k, v in kwargs.items()]
arg_str = ', '.join(arg_list)
print(f'{func.__name__}({arg_str}) = {result!r}')
return result
return out
```
Basic Script Template
---------------------
```python
#!/usr/bin/env python3
#
# Usage: .py
#
from collections import namedtuple
from enum import Enum
import re
import sys
def main():
pass
###
## UTIL
#
def read_file(filename):
with open(filename, encoding='utf-8') as file:
return file.readlines()
if __name__ == '__main__':
main()
```