python-cheatsheet/README.md

Python and Libraries Cheatsheet
===============================

Main
----
```python
if __name__ == '__main__':
    main()
```


List
----
```python
<list>[<inclusive from>:<exclusive to>:<step size>]
<list>.extend(<list>)
<list>.sort()
<list>.reverse()
sum(<list>)
sorted_by_second = sorted(<list>, key=lambda tup: tup[1])
```

Dictionary
----------
```python
<dict>.items()
<dict>.get(<key>, <default>)
<dict>.setdefault(<key>, <default>)
<dict>.update(<dict>)
collections.defaultdict(<type>)  # Creates list 
```

Init from two lists
```
dict(zip(keys, values))
```

Counter
```
>>> from collections import Counter
>>> z = ['blue', 'red', 'blue', 'yellow', 'blue', 'red']
>>> Counter(z)
Counter({'blue': 3, 'red': 2, 'yellow': 1})
```

Set
---
```python
<set> = set()
<set>.add(<el>)
<set>.update(<set>)
<set>.union(<set>)
<set>.intersection(<set>)
<set>.difference(<set>)
<frozenset> - is hashable can be used as key in dictionary
```

Range
-----
```python
range(<to exclusive>)
range(<from inclusive>, <to exclusive>)
range(<from inclusive>, <to exclusive>, <step size>)  # Negative step for backward
```

Enumerate
---------
```python
for i, <el> in enumerate(<list>, [start])
```

Named Tuples
------------
```
>>> TestResults = collections.namedtuple('TestResults', ['filed', 'attempted'])
>>> TestResults(1, 2)
TestResults(filed=1, attempted=2)
```


Iterator
--------
Reads input until it reaches empty line.
```
for line in iter(input, ''):
    print(line)
```
Use partial from functools if function needs arguments.

Skips first element.
```
next(<iter>)
for element in <iter>:
    ...
```

Type
----
```python
type(<el>) is int/str/set/list
```
```python
import numbers
isinstance(<el>, numbers.Number)
```


String
------
```python
str.replace(<text>, <old>, <new>)
<str>.isnumeric()
<str>.split()
<str>.strip()
'<str>'.join(<list>)
```

### Print
```python
print(<el1>, <el2>, end='', sep='', file=<file>)
```

### Regex
```python
import re
re.sub(<regex>, <new>, <text>)
re.search(<regex>, <text>)
```

### Format
```python
'{}'.format(<el>)
```

```python
{:<min width>}  -> '<el>    '
{:><min width>} -> '    <el>'
{:^<min width>} -> '  <el>  '
{:_<min width>}  -> '<el>____'
{:.<max width>} -> '<e>'
{:<max widht>.<min width>} -> '    <e>'
{:<max width>.<no of decimals>f} -> '  3.14'
```

```python
>>> person = {'name': 'Jean-Luc', 'height': 187.1}
>>> '{p[height]:.0f}'.format(p=person)
'187'
```

### Text Wrap
```python
import textwrap
textwrap.wrap(<text>, <width>)
```


Random
------
```python
import random
random.random()
random.randint(<from inclusive>, <to inclusive>)
random.shuffle(<list>)
```

Infinity
--------
```python
float("inf")
```

Datetime
--------
```python
import datetime
now = datetime.datetime.now()
now.strftime('%Y%m%d')
now.strftime('%Y%m%d%H%M%S')
```

Arguments
---------
```
args = (1, 2)
kwargs = {'x': 3, 'y': 4, 'z': 5}
func(*args, **kwargs)
# same as
func(1, 2, x=3, y=4, z=5)
```

* is the "splat" operator: It takes a list as input, and expands it into actual positional arguments in the function call.

So if uniqueCrossTabs was [ [ 1, 2 ], [ 3, 4 ] ], then itertools.chain(*uniqueCrossTabs) is the same as saying itertools.chain([ 1, 2 ], [ 3, 4 ])


Inline
------
### Lambda
```python
lambda <arg1>, <arg2>: <return value>
lambda: <return value>
```

### Comprehension
```python
[i+1 for i in range(10)]
[i for i in range(10) if i>5]
{i: i*2 for i in range(10)} - dictionary
(x+5 for x in range(0, 10)) - generator
```

```
[i+j for i in range(10) for j in range(10)]
# Same as:
for i in range(10):
    for j in range(10):
        out.append(i+j)
```

### Map, Filter, Reduce
```python
map(lambda x: x+1, range(10))
filter(lambda x: x>5, range(10))
functools.reduce(combining_function, list_of_inputs)
```

### Any, All
```
any(a[1] for a in aaa)
```

### If - Else
```
expression_if_true if condition else expression_if_false
```

Closure
-------
```python
def mult_clos(x):
   def wrapped(y):
            return x * y
    return wrapped 

mul_by_3 = mult_clos(3)
```

or
```
from functools import partial
partial(<function>, <parameter>)
```

Decorator
---------
```python
@closure_name
def function_that_gets_passed_to_closure():
    ...
```

Debugger example
```
from functools import wraps

def debug(func):
    @wraps(func)  # Nedded for metadata copying (func name, etc)
    def wrapper(*args, **kwargs):
        print(func.__name__)
        return func(*args, **kwargs)
    return wrapper

@debug
def add(x, y):
    return x + y
```


Generator
---------
```python
def step(start, step):
    while True:
        yield start
        start += step

stepper = step(10, 2)
next(stepper)
```


Class
-----
### Class
```python
class <name>:
    def __init__(self, <arg>):
        self.a = <arg>
    def __repr__(self):
        return str({'a': self.a})
    def __str__(self):
        return str(self.a)
```

### Enum
```python
import enum
class <name>(enum.Enum):
    <value> = <index>  # or enum.auto()
```

### Copy
```python
import copy
copy.copy(<object>)
copy.deepcopy(<object>)
```


System
------

### Arguments
```python
import sys
sys.argv
```

### Read File
```python
with open(<filename>, encoding='utf-8') as file:
    return file.readlines()
```
```
def get_file_contents(file_name):
    with open(file_name, encoding='utf-8') as f:
        return f.readlines()
```

### Write to File
```python
with open(<filename>, 'w', enconding='utf-8') as file:
    file.write(<text>)
```

### Execute Command
```python
import os
os.popen(<command>).read()
```

### Input
```python
filename = input('Enter a file name: ')
```

Print lines until EOF
```
while True:
    try:
        print(input())
    except EOFError:
        break
```

JSON
----
```python
import json
```

### Read File
```python
with open(<filename>, encoding='utf-8') as file:
    return json.load(file)
```

### Write to File
```python
with open(<filename>, 'w', enconding='utf-8') as file:
    file.write(json.dumps(<object>))
```



SQLite
------
```python
import sqlite3
db = sqlite3.connect(<filename>)
```

### Read
```python
cursor = db.execute(<query>)
if cursor:
    cursor.<fetchone/fetchall>()
db.close()
```

### Write
```python
db.execute(<query>)
db.commit()
```


Exceptions
----------
```
while True:
    try:
        x = int(input("Please enter a number: "))
        break
    except ValueError:
        print("Oops!  That was no valid number.  Try again...")
```

Threading
---------
```python
import threading
```

### Thread
```python
thread = threading.Thread(target=<function>, args=(<first arg>, ))
thread.start()
thread.join()
```

### Lock
```python
lock = threading.Rlock()
lock.acquire()
lock.release()
```

Itertools
---------
Every function returns an generator and can accept any collection.
All examples should be passed to list() to get the output.

```python
from itertools import *
```

### Chain
```python
>>> chain([1,2], range(3,5))
[1, 2, 3, 4]
```

### Combinations
```python
>>> combinations("abc", 2)
[('a', 'b'), ('a', 'c'), ('b', 'c')]
```

### Permutations
```python
>>> permutations("abc", 2)
[('a', 'b'), ('a', 'c'), ('b', 'a'), ('b', 'c'), ('c', 'a'), ('c', 'b')]
```

### Product
```python
>>> list(product('ab', [1,2]))
[('a', 1), ('a', 2), ('b', 1), ('b', 2)]
```

### Compress
```python
>>> compress("abc", [True, 0, 23])
['a', 'c']
```

### Count
```python
>>> a = count(5, 2)
>>> next(a), next(a)
(5, 7)
```

### Cycle
```python
>>> a = cycle("abc")
>>> [next(a) for _ in range(10)]
['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c', 'a']
```

### Groupby
```python
>>> {k: list(v) for k, v in groupby("aabbbc")}
{'a': ['a', 'a'], 'b': ['b', 'b', 'b'], 'c': ['c']}
```

```python
>>> a = [{"id": 1, "name": "bob"}, {"id": 2, "name": "bob"}, {"id": 3, "name": "peter"}]
>>> {k: list(v) for k, v in groupby(a, key=lambda x: x["name"])}
{'bob': [{'id': 1, 'name': 'bob'}, {'id': 2, 'name': 'bob'}], 'peter': [{'id': 3, 'name': 'peter'}]}
```

### Islice
```
islice([1, 2, 3], 1, None)
[2, 3]
```


### ifilter/imap/izip
Filter, map and zip functions that return generators instead of iterators


Introspection and Metaprograming
--------------------------------
Inspecting code at runetime and code that generates code.

```python
>>> class B:
...     def __init__(self):
...             self.a= 'sdfsd'
...             self.b = 123324
>>> b = B()
```

### Getattr
```python
>>> getattr(b, 'a')
'sdfsd'
```

same as
```
B.__getattribute__(b, 'a')
```

### Hasattr
```python
>>> hasattr(b, 'c')
False
```

### Setattr
```python
>>> setattr(b, 'c', 10)
```

Type
----
Type is the root class. If only passed the object it returns it's type.
Otherwise it creates new class (and not the instance!).
```python
type(class_name, parents[tuple], attributes[dict])
```

```python
BB = type('B', (), {'a': 'sdfsd', 'b': 123324}
b = BB()
```

MetaClass
---------
Classes that creates classes.
```python
def my_meta_class(name, parents, attrs):
    ... do stuff
    return type(name, parents, attrs)
```
or
```python
class MyMetaClass(type):
    def __new__(klass, name, parents, attrs):
        ... do stuff
        return type.__new__(klass, name, parents, attrs)
```

Do Stuff
--------
* Look at the attributes
* Set new attributes
* Create functions dynamically
* Traverse the parent classes
* Change values in the class

Metaclass Attr
--------------
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
```

Eval
----
```
import ast
import operator as op

# supported operators
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 eval_expr(expr):
    """
    >>> eval_expr('2^6')
    4
    >>> eval_expr('2**6')
    64
    >>> eval_expr('1 + 2*3**(4^5) / (6 + -7)')
    -5.0
    """
    print(expr)
    return eval_(ast.parse(expr, mode='eval').body)

def eval_(node):
    if isinstance(node, ast.Num): # <number>
        return node.n
    elif isinstance(node, ast.BinOp): # <left> <operator> <right>
        return operators[type(node.op)](eval_(node.left), eval_(node.right))
    elif isinstance(node, ast.UnaryOp): # <operator> <operand> e.g., -1
        return operators[type(node.op)](eval_(node.operand))
    else:
        raise TypeError(node)
```


Ascii
-----
Get char from int.
```
chr(<int>)
```

Flatten List
------------
```
[item for sublist in a_list for item in sublist]
```


Libraries
=========


Plot
----
```
import matplotlib
matplotlib.pyplot.plot(<data>, ...)
matplotlib.pyplot.show()
matplotlib.pyplot.savefig(<filename>)
```


Web
---
```
import bottle
```

### Run
```
bottle.run(host='localhost', port=8080)
bottle.run(host='0.0.0.0', port=80, server='cherypy')
```

### Static request

### Dynamic request

### REST request


Curses
------
```
import curses
def main():
    curses.wrapper(draw)
def draw(screen):
    screen.clear()
    screen.addstr(0, 0, "Press ESC to quit.")
    while screen.getch() != 27:
        pass
```



Profile
-------
```
import timeit
timeit.timeit('"-".join(str(n) for n in range(100))', number=10000)
```

```
import pycallgraph
graph = pycallgraph.output.GraphvizOutput()
graph.output_file = <filename>
with pycallgraph.PyCallGraph(output=graph):
    <code>
```