Effective Python

classes.py

def log_missing():
   print('Key added')
   return 0
   
current = {'green': 12, 'blue': 3}
increments = [('red', 5), ('blue', 17), ('orange', 9)]

result = defaultdict(log_missing, current)
print('Before:', dict(result))
for key, amount in increments:
    result[key] += amount
print('After: ', dict(result))

>>>
Before: {'green': 12, 'blue': 3}
Key added
Key added
After:  {'orange': 9, 'green': 12, 'blue': 20, 'red': 5}

def increment_with_report(current, increments):
    added_count = 0

    def missing():
      nonlocal added_count  # Stateful closure
      added_count += 1
      return 0

    result = defaultdict(missing, current)
    for key, amount in increments:
        result[key] += amount

    return result, added_count

class BetterCountMissing(object):
    def __init__(self):
        self.added = 0
        
    def __call__(self):
        self.added += 1
        return 0

counter = BetterCountMissing()
result = defaultdict(counter, current)  # Relies on __call__
for key, amount in increments:
    result[key] += amount
assert counter.added == 2

# Gettattr demo
class LazyDB(object):
    def __init__(self):
        self.exists = 5

    def __getattr__(self, name):
        value = 'Value for %s' % name
        setattr(self, name, value)
        return value”

class LoggingLazyDB(LazyDB):
    def __getattr__(self, name):
        print('Called __getattr__(%s)' % name)
        return super().__getattr__(name)

data = LoggingLazyDB()
print('exists:', data.exists)
print('foo:   ', data.foo)
print('foo:   ', data.foo)

>>>
exists: 5
Called __getattr__(foo)
foo:    Value for foo
# Because of setattr called now __getattr__ isn't calling
foo:    Value for foo

functions.py

def sort_priority(values, group):
    def helper(x):
        if x in group:
            return (0, x)
        return (1, x)
    values.sort(key=helper)

iterators.py

it = read_visits('/tmp/my_numbers.txt')
print(list(it))
print(list(it))  # Already exhausted

>>>
[15, 35, 80]
[]

python_thinking.py

even_squares = [x**2 for x in a if x % 2 == 0]


chile_ranks = {'ghost': 1, 'habanero': 2, 'cayenne': 3}
rank_dict = {rank: name for name, rank in chile_ranks.items()}
chile_len_set = {len(name) for name in rank_dict.values()}


matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [x for row in matrix for x in row]

it = (len(x) for x in open('/tmp/my_file.txt'))