walrus_operator.py

# Walrus Operator
# Hello, Walrus!
# To get a first impression of what assignment expressions are all about,
# start your REPL and play around with the following code:
walrus = False
print(walrus)

(walrus := True)
print(walrus)


# Dictionaries
numbers = [2, 8, 0, 1, 1, 9, 7, 7]

'''Note that both the sum and the length of the numbers list are calculated twice.'''
description = {
    "length": len(numbers),
    "sum": sum(numbers),
    "mean": sum(numbers) / len(numbers),
}
print(description)

'''The variables num_length and num_sum are only used to optimize the calculations
inside the dictionary. By using the walrus operator, you can make this role clearer'''
description = {
    "length": (num_length := len(numbers)),
    "sum": (num_sum := sum(numbers)),
    "mean": num_sum / num_length,
}
print(description)


# List
'''Here, you filter the numbers list and leave the positive results from applying slow().
The problem with this code is that this expensive function is called twice.
'''
numbers = [7, 6, 1, 4, 1, 8, 0, 6]


def slow(x):
    return x-2


results = [slow(num) for num in numbers if slow(num) > 0]
print(results)

'''You can rewrite the list comprehension using the walrus operator as follows,
Note that the parentheses around value := slow(num) are required.
This version is effective and readable, and it communicates the intent of the code well.'''
results = [value for num in numbers if (value := slow(num)) > 0]
print(results)


'''Here, you first capture all city names that start with "B".
Then, if there’s at least one such city name, you print out the
first city name starting with "B".'''
cities = ['Monaco', 'Vancouver', 'Rio de Janeiro', 'Berlin', 'La Paz', 'Bogota', 'Dallas']

if any((witness := city).startswith("B") for city in cities):
    print(f"{witness} starts with B")
else:
    print("No city name starts with B")

'''You can more clearly see what’s happening by wrapping .startswith("B") in a function that
also prints out which item is being checked:'''


def starts_with_b(name):
    print(f"Checking {name}: {(result := name.startswith('B'))}")
    return result


any(starts_with_b(city) for city in cities)
all(starts_with_b(city) for city in cities)