Day 4#

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Chapter 4: Input, Output, and Formatting#

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4.1 The print() Function#

print() displays values on screen. It is Python's primary output tool.

Basic Usage#

print("Hello, world!")
print(42)
print(3.14)
print(True)

Multiple Arguments#

print() can accept multiple arguments separated by commas. By default it prints them separated by a space:

name = "Alice"
age = 30
print("Name:", name, "Age:", age)
# Output: Name: Alice Age: 30

The sep Parameter#

You can change the separator between arguments using sep:

print("a", "b", "c")           # a b c
print("a", "b", "c", sep="")   # abc
print("a", "b", "c", sep="-")  # a-b-c
print("a", "b", "c", sep="\n") # a
                                 # b
                                 # c

The end Parameter#

By default, print() ends each output with a newline character (\n). You can change this:

print("Hello", end=" ")
print("World")
# Output: Hello World    (all on one line)

print("A", end="")
print("B", end="")
print("C")
# Output: ABC

Escape Characters#

Inside strings, certain two-character sequences have special meaning:

print("Line 1\nLine 2")     # prints on two lines
print("Column1\tColumn2")   # tab between them
print("She said \"hi\"")    # She said "hi"

4.2 The input() Function#

input() pauses the program, displays a prompt, waits for the user to type something and press Enter, then returns what they typed as a string.

name = input("What is your name? ")
print("Hello,", name)

Critical point: input() always returns a string. Even if the user types a number, input() gives you the string "42", not the integer 42. This is the source of many beginner bugs.

Converting Input to Numbers#

If you need a number from the user, wrap input() in a type conversion:

# WRONG — this will cause errors when you try to do math:
age = input("Enter your age: ")      # age is a string like "25"
age_next_year = age + 1              # TypeError!

# RIGHT — convert to int first:
age = int(input("Enter your age: "))  # age is now an integer
age_next_year = age + 1               # Works!
print("Next year you'll be", age_next_year)

Similarly for floats:

price = float(input("Enter the price: "))
tax = price * 0.1
print("Total with tax:", price + tax)

What Happens if the User Types Something Unexpected?#

If the user types "hello" when you call int(input(...)), Python raises a ValueError. We'll learn to handle this gracefully in Week 7. For now, assume the user enters valid input.

4.3 String Formatting with f-Strings#

F-strings (formatted string literals) are the modern, readable way to build strings that incorporate variable values.

Basic Syntax#

An f-string starts with f immediately before the opening quote:

name = "Alice"
age = 30
print(f"My name is {name} and I am {age} years old.")
# Output: My name is Alice and I am 30 years old.

Inside an f-string, anything inside {} is evaluated as a Python expression and converted to a string:

x = 5
y = 3
print(f"{x} times {y} is {x * y}")    # 5 times 3 is 15

Any expression works inside the braces:

print(f"The square of 7 is {7 ** 2}")       # 49
print(f"Type of pi: {type(3.14159)}")        # <class 'float'>
print(f"Length of 'hello': {len('hello')}")  # 5

Format Specifiers#

After a colon inside the braces, you can add formatting instructions:

pi = 3.14159265358979

# Control decimal places:
print(f"Pi is approximately {pi:.2f}")   # Pi is approximately 3.14
print(f"Pi is approximately {pi:.5f}")   # Pi is approximately 3.14159

# Add thousands comma:
big_number = 1234567
print(f"Big number: {big_number:,}")     # Big number: 1,234,567

# Control width and alignment:
print(f"{'Name':>10} {'Score':>6}")     # right-align in 10/6 char field
print(f"{'Alice':>10} {95:>6}")
print(f"{'Bob':>10} {87:>6}")

# Percentage:
ratio = 0.8532
print(f"Accuracy: {ratio:.1%}")          # Accuracy: 85.3%

Format specifier syntax: {value:formatspec} where formatspec is one of:

• .Nf — float with N decimal places
• , — add thousands comma
• .N% — percentage with N decimal places
• >N — right-align in N characters
• <N — left-align in N characters
• ^N — center in N characters

The str() Function#

str() converts any value to its string representation:

x = 42
s = "The answer is " + str(x)    # str() needed for concatenation
print(s)    # The answer is 42

In f-strings, the conversion happens automatically. But when using + to build strings, str() is often needed.

4.4 A Note on print() vs. f-strings#

There are two common ways to display formatted output:

name = "Alice"
score = 95.5

# Method 1: print() with comma-separated arguments
print("Name:", name, "Score:", score)

# Method 2: f-string
print(f"Name: {name}  Score: {score:.1f}")

Method 1 is simple but gives less control over formatting. Method 2 is more flexible and reads more naturally. In this course, prefer f-strings for any output that involves mixing text and variables.