# The special value for missing data in Python is called None
my_value = None
print(my_value)

# Check if a variable is missing with is operator
value = None
if value is None:
    print('The value is missing.')
else:
    print('The value is present.')
    

# User input and missing values
user_age = input('Enter your age, or press Enter to skip: ')
if user_age == '':
    user_age = None
print('User age:', user_age)

# Detecting missing entries in a list
data = [45, None, 35, None, 65]
for item in data:
    if item is None:
        print('Missing entry found!')
    else:
        print('Entry is:', item)
        

# Counting missing values in a list
scores = [55, None, 67, 89, None, 40]
missing_count = 0
for score in scores:
    if score is None:
        missing_count += 1
print('There are', missing_count, 'missing entries.')

# Replacing missing values with a default
temperatures = [22, None, 19, None, 21]
for i in range(len(temperatures)):
    if temperatures[i] is None:
        temperatures[i] = 20
print('Updated temperatures:', temperatures)

# Remove missing values from a list
data_with_gaps = [3, None, 8, None, 2]
cleaned = []
for item in data_with_gaps:
    if item is not None:
        cleaned.append(item)
print('Clean data:', cleaned)

# Find the first non-missing value
answers = [None, None, 5, 7]
for answer in answers:
    if answer is not None:
        print('First answer is:', answer)
        break
    

# Example: Handling missing values in dictionary
info = {'name': 'Sam', 'birthday': None, 'score': 88}
if info['birthday'] is None:
    print('Birthday is missing!')
    

# Safe dictionary lookup with get()
contacts = {'Dana': '555-1234'}
phone = contacts.get('Alex', None)
if phone is None:
    print('Phone number for Alex is missing.')
    

# Handling missing data in real-world text
text = input('Enter your city, or press Enter if missing: ')
if text == '':
    city = None
else:
    city = text
print('City is:', city)

# Calculate average, ignoring missing numbers
grades = [90, None, 85, None, 78]
total = 0
count = 0
for grade in grades:
    if grade is not None:
        total += grade
        count += 1
if count > 0:
    average = total / count
    print('Average grade:', average)
else:
    print('No grades to average.')
    

# List comprehensions to filter missing data
data = [10, None, 30, None, 50]
cleaned = [x for x in data if x is not None]
print('Values without missing data:', cleaned)

# Sorting data, putting missing values at the end
results = [25, None, 15, 40, None, 30]
sorted_results = sorted(results, key=lambda x: (x is None, x))
print('Sorted data:', sorted_results)

# Filtering out outliers and missing data
numbers = [100, None, 150, 20, 300, None, 40]
filtered = [x for x in numbers if x is not None and x < 200]
print('Filtered data:', filtered)

# Mini-project, part 1: Clean favorite colors
responses = ['blue', None, 'green', '', 'red', None, '', 'blue']
cleaned = []
for color in responses:
    if color and color is not None:
        cleaned.append(color)
print('Clean responses:', cleaned)

# Mini-project, part 2: Count answers
color_count = {}
for color in cleaned:
    if color not in color_count:
        color_count[color] = 1
    else:
        color_count[color] += 1
print('Answer counts:', color_count)

# Best practice: Document missing values with comments
data = [12, None, 18]
# None means 'value not recorded' for this data.
print(data)

# What if you forget to check for missing data?
values = [5, None, 10]
try:
    total = 0
    for v in values:
        total += v
    print('Total:', total)
except TypeError:
    print('Error: Tried to add a missing value!')
    

# Quick tip: Use or to set a default if missing
result = None
output = result or 'Unknown'
print('Result is:', output)