# 如果调用 cylinder_volume 函数时，不提供radius参数，那么radius的值为5
def cylinder_volume(height, radius=5):
  pi = 3.14159
  return height * pi * radius ** 2

cylinder_volume(10, 7) # 1539.3791
cylinder_volume(height=10, radius=7) # 1539.3791
cylinder_volume(radius=7, height=10) # 1539.3791

def population_density(population, land_area):
  return population / land_area
test1 = population_density(10, 1)
expected_result1 = 10
print("expected result: {}, actual result: {}".format(expected_result1, test1)) # expected result: 10, actual result: 10.0
test2 = population_density(864816, 121.4)
expected_result2 = 7123.6902801
print("expected result: {}, actual result: {}".format(expected_result2, test2)) # expected result: 7123.6902801, actual result: 7123.690280065897

def readable_timedelta(days):
  weeks = int(days / 7)
  day = days % 7
  return str(weeks) + ' week(s) and ' + str(day) + ' day(s).'
print(readable_timedelta(10)) # 1 week(s) and 3 day(s).

# This will result in an error
def some_function():
  word = "hello"
print(word)

def some_function():
  word = "hello"
def another_function():
  word = "goodbye"

word = "hello"
def some_function():
  print(word)
print(word)

egg_count = 0
def buy_eggs():
  egg_count += 12 # purchase a dozen eggs
buy_eggs()