Python REPL reference

What is Python REPL?

The Python interpreter allows for what’s known as an interactive REPL (Read-Eval-Print Loop), or shell, which

Reads a piece of code

Evaluates it, and then

Prints the result to the console in a

Loop

The Python interpreter can execute Python code in two modes:

Script, or program

Interactive, or REPL

The main difference between a module and a script is that modules are meant to be imported, while scripts are made to be directly executed.

Interactive mode

When you run the Python interpreter in interactive mode, you open an interactive shell, also known as an interactive or a REPL session. In this shell, your keyboard is the input source, and your screen is the output destination.

Why Use a Python REPL?

As a Python programmer, you’ll spend considerable time in interactive mode.

Explore and learn Python syntax

Try out and prove ideas, concepts, and implementations

Starting and Ending REPL Interactive Sessions

1. Running the python Command Open command-line window. Run python without any arg. It shows current Python version and platform you are running the interpreter.

The second line shows a message with commands that you can run to get additional information about Python in general.

The last line is primary prompt of a standard Python interactive session or shell.

The interpreter also has a secondary prompt represented by three dots (...).

>>> number = 42

>>> assert isinstance(number, int) and number > 0, \
...     f"number greater than 0 expected, got: {number}"

2. Passing Command-Line Options to the python Command

• -i flag option makes the interpreter enter the interactive mode after running a script
• -c option executing a piece of code
• -m creating virtual environment

C:\>python -c "print('Hello word')"

C:\REPL>python -i sample.py

Check the current global variables in your script.

globals()

Inspect the stack trace when your program raises an exception.

Call mean() with an empty list as an argument.

mean([])

In this case, the function fails with a ZeroDivisionError because calling len() with an empty list returns 0.

Task 1 Script mode

In Python interpreter:

Execute script sample.py in Python interpreter.

Write your own script, execute the function random from random module, print the ranodm number to the screen.

• -b option when you’re running code that compares bytes objects

Run python with the -b option and without.

>>> my_str = "Welcome to Python"
>>> bytes_str = b"Hello"
>>> my_str == bytes_str

Understanding bytes. Check it on your name.

bytes_str = b"Hello"
>>> for byte in bytes_str:
...     print(byte,end='')
...     print('\n')

Python bytes() function is used to convert an object to an immutable(cannot be modified) byte object of the given size and data. The Python bytes() function returns a byte's object, which is an immutable series of integer numbers ranging from 0 to 256.

3. Exiting the Current Python REPL Session

• quit()
• exit()
• Ctrl+Z and then Enter on Windows systems
• Ctrl+D on Unix systems, such as Linux or macOS

These key combinations represent the end-of-file character (EOF) in the corresponding OS.

Both functions allow you to exit the current session by implicitly raising a SystemExit exception.

They allow you to exit the current interactive session because the interpreter runs in a special file called main, as you can confirm by inspecting the name attribute:

>>> __name__

All the Python code in an interactive session will be contained in the main file, which runs until an EOF character is read.

This means that when the interpreter finds this character, it immediately terminates the current REPL session.

Running Code in a REPL Session

1. Simple statement

When the REPL’s primary prompt (>>>) appears on your screen, type in the following expressions, pressing the Enter key after each of them:

>>> 5 - 2
3

>>> sum([1, 2, 3, 4])
10

>>> 42 > 7
True

>>> number = 42

>>> 7 / 0

While running these examples, note how after executing each expression, the interpreter loops back to the primary prompt (>>>), which lets you introduce a new expression. Now you’ve seen the REPL cycle in action. The Python interpreter has read your expressions, executed them, and printed their corresponding result to finally loop back to the primary prompt.

2. Compund statements Conditionals, loops, and with statements

>>> number = -42

if number < 0:
    print("negative")
elif number > 0:
    print("positive")
else:
    print("equal to 0")

When it comes to entering indented code blocks, keep in mind that the standard REPL doesn’t support auto-indentation:

3. Dealing With Explicit and Implicit Line Continuations

Explicitly join multiple physical lines into a single logical line using the backslash (\) character

>>> number = 42

>>> assert isinstance(number, int) and number > 0, \
...     f"number greater than 0 expected, got: {number}"

Once you open a bracket, such as [], (), or {}, and press Enter, you get the REPL’s secondary prompt. This is known as implicit line joining.

4. Printing vs Evaluating

The interpreter doesn’t display anything for statements that don’t generate return values.

>>> numbers = [2, 4, 1, 3]

>>> numbers.sort()
>>> numbers.append(5)

Because these method calls don’t issue any output to the screen, it may seem that they didn’t perform any real action. However, they did.

>>> print(numbers.append(6))

5. Flagging and understanding errors

When an error occurs in a REPL session, the interpreter automatically prints the corresponding error message and traceback. Then, it loops back to the REPL’s primary prompt.

Explore the follwoing errors:

>>> greeting = "Hello, World!
>>> 42 / 0
>>> 42 / 0

6. Using the _ Special Variable

>>> 42 < 7
>>> _

7. Reloading imported modules

>>> import greeting

>>> greeting.greet()
'Hello, World!'
>>> greeting.greet("Pythonista")

TASK 2 implement additional functionality and test it.

Now say that you want to add a new argument to your function. The argument will be a Boolean flag that allows printing the greeting message in uppercase letters.

Handle the list of names instead of str.

>>> import importlib
>>> importlib.reload(greeting)