Interacting with the user via the console is a fundamental part of writing terminal scripts and CLI applications.

---

1. input() function

Python allows for command line input. That means we are able to ask the user for input. The method is input() (in Python 2.x it was raw_input()).

username = input("Enter username: ")
print("Username is: " + username)

Important: The input() function always returns a string. If you expect the user to enter a number to do math, you must explicitly cast it.

age = input("Enter your age: ")
# Convert to integer
age_int = int(age)

print(f"Next year you will be {age_int + 1}")

---

2. print() function (sep, end, flush)

You are already familiar with the print() function, which outputs data to the standard output device (the screen).

However, print() has a few hidden parameters that are very useful:

sep (Separator)

Specify how to separate the objects, if there is more than one. Default is a space ' '.

print("apple", "banana", "cherry")
# Outputs: apple banana cherry

print("apple", "banana", "cherry", sep="-")
# Outputs: apple-banana-cherry

end

Specify what to print at the end. Default is '\n' (line feed/newline).

# Prints on two lines
print("Hello")
print("World")

# Prints on the same line
print("Hello", end=" ")
print("World")
# Outputs: Hello World

flush

A boolean, specifying if the output is flushed (True) or buffered (False). Default is False. This is useful when you are printing progress bars and need the terminal to update immediately.

---

3. Formatting output

We covered string formatting earlier, but it is deeply tied to I/O.

When printing out variables to the user, the modern f-string approach is highly recommended. You can also perform inline calculations and formatting inside the f-string.

price = 49.99
tax = 0.05

# Display price with exactly 2 decimal places
print(f"The total cost is: ${price + (price * tax):.2f}")

The :.2f modifier tells Python to format the floating point number to 2 decimal places.