Python - lambda, list comp i map/filter/reduce

class: center, middle

# Python

## lambda, list comp i map/filter/reduce

### 07/12/2018

Paweł Suder

<pasuder@agh.edu.pl>

https://pasuder.github.io/labs/doc/lab03

---

# Agenda

1. Domknięcia
1. Dekoratory
1. _List comprehension_
1. `lambda`
1. `map`/`filter`/`reduce`
1. Zadania

---

### Domknięcia

```python
def print_func(text_before_text):
    def inner_print(text):
        print("%s: %s" % (text_before_text, text))
    return inner_print
```

```python
>>> print_func('Provided text')
<function print_func.<locals>.inner_print at 0x7f7064828950>
>>> print_text = print_func('Provided text')
>>> print_text('that text')
Provided text: that text
```

---

### Domknięcia

```python
global_text = 'Global text'
def print_func(text_before_text):
    def inner_print(text):
        print("%s: %s: %s" % (global_text, text_before_text, text))
    return inner_print
```

```python
>>> print_func('Provided text')
<function print_func.<locals>.inner_print at 0x7f70648e3e18>
>>> print_text = print_func('Provided text')
>>> print_text('that text')
Global text: Provided text: that text

>>> global_text = 'Another global text'
>>> print_text('that text')
Another global text: Provided text: that text
```

---

### Dekoratory

```python
def wrap_func(func):
    def inner_func(*args, **kwargs):
        try:
            print("Wrapping function '%s' call with args: %s; and "
                  "kwargs: %s" % (func.__name__, args, kwargs))
            return func(*args, **kwargs)
        finally:
            print('End of wrap')
    return inner_func

@wrap_func
def print_text(text, start_text='Provided text'):
    print("%s: %s" % (start_text, text))
```

---

### Dekoratory

```python
>>> print_text('blah')
Wrapping function 'print_text' call
  with args: ('blah',); and kwargs: {}
Provided text: blah
End of wrap

>>> print_text('blah', 'meh')
Wrapping function 'print_text' call 
  with args: ('blah', 'meh'); and kwargs: {}
meh: blah
End of wrap

```

---

### Dekoratory

```python
>>> print_text('blah', start_text='meh')
Wrapping function 'print_text' call 
  with args: ('blah',); and kwargs: {'start_text': 'meh'}
meh: blah
End of wrap

>>> print_text(text='blah', start_text='meh')
Wrapping function 'print_text' call 
  with args: (); and kwargs: {'text': 'blah', 'start_text': 'meh'}
meh: blah
End of wrap
```

---

### List comprehension

```python
>>> [a for a in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> [a+1 for a in range(10)]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

>>> [a*2 for a in range(10)]
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

>>> type([a for a in range(10)])
<class 'list'>
```

---

### List comprehension

```python
>>> (a*2 for a in range(10))
<generator object <genexpr> at 0x7f706482ef68>

>>> for i in (a*2 for a in range(10)):
...     print(i, end=' ')
... 
0 2 4 6 8 10 12 14 16 18

>>> {a*2 for a in range(10)}
{0, 2, 4, 6, 8, 10, 12, 14, 16, 18}

>>> type({a*2 for a in range(10)})
<class 'set'>
```

---

### List comprehension

```python
>>> {str(a): a*2 for a in range(10)}
{'0': 0, '1': 2, '2': 4, '3': 6, '4': 8, 
 '5': 10, '6': 12, '7': 14, '8': 16, '9': 18}

>>> type({str(a): a*2 for a in range(10)})
<class 'dict'>
```

---

### lambda

```python
>>> add_two = lambda a: a+2
>>> add_two(3)
5

>>> add = lambda a, b: a+b
>>> add(1, 3)
4
```

---

### map/filter/reduce

```python
>>> map(lambda a: a+2, range(10))
<map object at 0x7f65c60ab7f0>

>>> list(map(lambda a: a+2, range(10)))
[2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
```

---

### map/filter/reduce

```python
>>> list(filter(lambda a: a%2, range(10)))
[1, 3, 5, 7, 9]

>>> list(filter(lambda a: a%2 == 0, range(10)))
[0, 2, 4, 6, 8]

>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
```

---

### map/filter/reduce

`reduce` nie jest słowem-kluczem od python3

```python
from functools import reduce
>>> reduce(lambda a, b: a + b, range(10), 0)
45
>>> sum(range(10))
45

>>> reduce(lambda a, b: a * b, range(10), 0)
0
>>> reduce(lambda a, b: a * b, range(10), 1)
0
>>> reduce(lambda a, b: a * b, range(1,10), 1)  # można pominąć 1
362880
>>> list(range(1,10))
[1, 2, 3, 4, 5, 6, 7, 8, 9]
```

---

### map/filter/reduce

```python
>>> def check_if_odd(i):
...     return i % 2

>>> list(filter(check_if_odd, range(10)))
[1, 3, 5, 7, 9]
```

---

### Więcej informacji

- [args i kwargs](http://book.pythontips.com/en/latest/args_and_kwargs.html)
- [Domknięcia](https://www.programiz.com/python-programming/closure)
- [Dekoratory](http://book.pythontips.com/en/latest/decorators.html)
- [Więcej o dekoratorach](https://realpython.com/primer-on-python-decorators/)
- [map](https://docs.python.org/3.5/library/functions.html#map)
- [filter](https://docs.python.org/3.5/library/functions.html#filter)
- [map/filter/reduce](http://book.pythontips.com/en/latest/map_filter.html)

---

### Zadania

Wykonaj:

- dekorator funkcji, który łapie wszystkie wyjątki z wywołanej funkcji i informuje, co za wyjątek został złapany
- filtrowanie tablicy liczby naturalnych do 100, sprawdzając, czy liczba jest liczbą pierwszą
- dodanie liczb naturalnych, nieparzystych, do 100

Pro-tip:

- `*args, **kwargs`
- `range()`