`Console`

In Python 3 we use input and output methods. With print(), which requires method syntax, we generate output. This is a simple, easy-to-maintain approach.

We can pass arguments to print() to specify what values to print, and how to print them. With no argument, we print an empty line.

Print

This program uses 3 calls to print(). It uses one and then 2 arguments. When 2 arguments are used to print, a space is rendered in between the strings.

And Print can handle variables. We show a string variable, but other values (like integers) and even objects can be printed.

# Print a string literal.
print("Hello")

# Print two arguments.
print("Hello", "there")

# Print the value of a string variable.
a = "Python"
print(a)Hello
Hello there
Python

Input

Here we read characters and strings (lines) from the console. When this program executes, it will pause and wait for a line input.

Then The user types some characters, such as "x" and presses enter. The input() method immediately returns this value.

Detail Often a loop is needed when designing console programs. This makes the program interactive.

# Get input from console.
s = input()
print("You typed", s)

# Use if-statement on input.
if s == "a":
    print("Letter a detected")
else:
    print("Not letter a")a
You typed a
Letter a detected

Interactive

A console program can continually prompt the user until a quit command is received. This allows the same program to run many commands without restarting.

Here We use a while-true loop and break if the user types a lowercase "q." The loop continues infinitely until a "q" is received.

# Continue while true.
while True:
    # Get input.
    value = input()

    # Break if user types q.
    if value == "q":
        break

    # Display value.
    print("You typed: ", value)

# Exit message.
print("You quit.")1
You typed:  1
2
You typed:  2
3
You typed:  3
q
You quit.

Convert

The result of the input method is a string. It can be converted to any other type. Here I convert the result to an integer with the int() built-in operator.

Tip When creating a program, it is best to use the data type that most closely matches your data.

print("Enter number:")
# Get input and convert it to an integer.
value = input()
number1 = int(value)

print("Again:")
value = input()
number2 = int(value)

print("Product:")
# Multiply the two numbers.
print(number1 * number2)Enter number:
10
Again:
2
Product:
20

Validate

User input is often imperfect. It has errors. We can use the try and except statements to validate this input. The int() method raises an error if the input is invalid.

So In this example we continually prompt the user (by passing a string argument to the input method).

And We convert the string received into an integer. If the value is not numeric, int() will raise an error. The loop will continue.

while True:
    try:
        # Get input with prompt.
        code = input("Code: ")
        # Attempt to parse input.
        value = int(code)
        break;
    except:
        print("Invalid code")

print("Value:", value)Code: g
Invalid code
Code: 89
Value: 89

Help

We can use Python in interactive mode. This helps us learn about Python constructs. In interactive mode, we can call the "help" built-in with an argument.

Detail This can be any term that Python can evaluate. Here I try the keyword "dict" which constructs a dictionary.

>>>> help(dict)

Help on class dict in module builtins:

class dict(object)
 |  dict() -> new empty dictionary.
 |  dict(mapping) -> new dictionary initialized from a mapping object's
 |      (key, value) pairs.

Print, file

By default, print() writes to the console. But we can change it to write to a file on the disk. We use the optional file argument.

Detail We can specify a file object. Often programs receive this object from the open() method.

Note When using this syntax, each call to print must specify the file argument. Otherwise, the default output (the console) is used.

Note 2 In most programs, using the file object directly leads to clearer code.

# Open this file for writing.
f = open("C:\\profiles\\perls.txt", "w")

# Print lines to the file.
print("Some text", file=f)
print("Some more text", file=f)Some text
Some more text

Print, end

Print has an optional "end" argument. This is set to a newline sequence by default, but you can override it. We often set end as a named argument.

Here In this example, we set end to an empty string literal. So print renders no end.

Tip By setting end to an empty string, we can use print multiple times on a single line.

Tip 2 We can avoid concatenating strings before displaying them in this way. Please also consider calling print() with multiple arguments.

# Change end argument to avoid newline.
print("Hi, ", end="")
print("how are you?")
Hi, how are you?

Print, no arguments

The print method can be used with no arguments. In this case, it will simply output the "end" value. By default, the end is a newline.

Here The output contains a blank line (with zero length) between the two numbers 100 and 200.

print(100)

# Use an empty print statement.
print()
print(200)100

200

Expressions

All expressions are evaluated before being passed to the print() method. Here, 1 + 2 is transformed to 3 before being passed to print.

Info To override this behavior, we need to construct a string out of our numbers before passing anything to print.

# This prints 3.
print(1 + 2)

# This prints the expression.
print("1 + 2")3
1 + 2

Format

Print does not support format strings. Instead we must pass the result of the str.format method to the print method. This syntax is not short, but it does make sense.

# Print formatted string.
value = 10
print(str.format("There are {} apples", value))
There are 10 apples

Repr

The __repr__ method on a class determines how it is printed. On your custom objects, you can provide an implementation for repr. Formatting can be done in one location only.

With print() and input(), we can write data to the console, and accept input from the user's keyboard. Many simple programs are best implemented with text interfaces.