Python Shell#

The Python Shell is a way to play around with Python code without needing to save a new file. It can be useful when you’re trying to figure things out. Think of it like a chalkboard – what you type will only be saved as long as the shell is open.

You can access it from a few places but here I want to show you how to use it from repl.it.

Part 1. Open the Python Shell #

At the command line type python and hit enter. The prompt will change to three >>>.

That means you’re in the Python Shell.

Listing 664 Python shell#

Python 2.7.17 (default, Apr 15 2020, 17:20:14)
[GCC 7.5.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

In these lessons we show Python shell examples in one of the following styles:

Listing 665 Python shell#

>>> print("like this...")
like this...

The caption Python Shell tells you that this is executed in the Python Shell. The >>> are input lines, and the rest are output.

This should look very similar to what you see in your own Python shell.

print("...or like that")

...or like that

The green border tells you that this code that you can edit live on this site (by clicking the icon at the top of the page), or type into your own Python shell.

The box labeled In is input code and the box labeled Out is output.

Part 2. Simple expressions #

Here you can type simple expressions to see what they will do. Here are a few things to try.

age = 28
age * 365

name = "Jayson"
clothing = "hat"
print("Hello " + name + ". What a nice " + clothing + " you are wearing!")

Hello Jayson. What a nice hat you are wearing!

It can be helpful to test out conditionals that you would want to put in an if statement.

False == 0

True

2.0 == 2

True

2 + 2 == 4

True

2 + 2 + 2 == 2 * 3

True

min_age = 21
age = 28
age >= min_age

True

Type some expressions of your own.

Part 3. The semicolon shorthand #

You can put two lines of code on the same line putting a ; where the line break would be. This is not recommended in your scripts but it can be a useful shorthand in the Python Shell.

For example, we could change the above to:

name = "Jayson" ; clothing = "hat"
print("Hello " + name + ". What a nice " + clothing + " you are wearing!")

Hello Jayson. What a nice hat you are wearing!

Part 4. Use Up-arrow and Down-arrow to repeat history #

You can use the up arrow to fill in previously typed lines in your history. Imagine we made a mistake.

Listing 666 Python shell#

>>> print("Hello world)
  File "<stdin>", line 1
    print("Hello world)
                      ^
SyntaxError: EOL while scanning string literal

We can hit the up-arrow once to bring the last line back, then fix the mistake and press enter.

Listing 667 Python shell#

>>> print("Hello world")
Hello world

Pressing the up-arrow more than once will bring back lines from further back in the history. The down arrow will bring us to more recent lines.

For example, we could use the up-arrow to bring back the line where we define the name and clothing variables and change their values. After pressing enter, we could use the up-arrow again to bring back the print line to repeat it with the new values.

Listing 668 Python shell#

>>> name = "Cody" ; clothing = "shirt"
>>> print("Hello " + name + ". What a nice " + clothing + " you are wearing!")
Hello Cody. What a nice shirt you are wearing!
>>> name = "Sean" ; clothing = "face"
>>> print("Hello " + name + ". What a nice " + clothing + " you are wearing!")
Hello Sean. What a nice face you are wearing!

Part 5. Tab completion #

To save typing time the Python Shell uses a nifty trick called tab completion. For identifiers that are either built-in or ones that we define, it we can type out just the first few characters then hit the Tab key, and it will attempt to guess the rest.

Try typing pri and then hit the tab key.

Listing 669 Python shell#

>>> print(

If there are multiple possible identifiers that could match those characters, tab-completion will fill in up to the place where they are different, then show a list of the matches. For example, if we had favorite_color and favorite_season defined, and then we typed fav followed by the tab key we would see:

Listing 670 Python shell#

>>> favorite_color = "black"
>>> favorite_season = "fall"
>>> fav
favorite_color   favorite_season
>>> favorite_

We could then type just the letter c then tab to have it fill in favorite_color.

Part 6. Inspect thyself #

Python has some helper functions that you can use to figure out more information about the things you have defined.

The type() function will tell you what type a variable is.

five = 5
type(five)

int

five = 5.0
type(five)

float

five = "5"
type(five)

str

type(print)

builtin_function_or_method

The callable() function will tell you if an object is a function or not.

callable(five)

False

callable(print)

True

Part 7. Get help #

The Python Shell includes a handy help() function that you can use to get more information about all sorts of things.

You can pass it a string with the name of a function you would like more information about.

help("print")

Help on built-in function print in module builtins:

print(...)
    print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)
    
    Prints the values to a stream, or to sys.stdout by default.
    Optional keyword arguments:
    file:  a file-like object (stream); defaults to the current sys.stdout.
    sep:   string inserted between values, default a space.
    end:   string appended after the last value, default a newline.
    flush: whether to forcibly flush the stream.

help("input")

Help on method raw_input in module ipykernel.kernelbase:

input = raw_input(prompt='') method of ipykernel.ipkernel.IPythonKernel instance
    Forward raw_input to frontends
    
    Raises
    ------
    StdinNotImplementedError if active frontend doesn't support stdin.

help("if")

The "if" statement
******************

The "if" statement is used for conditional execution:

   if_stmt ::= "if" assignment_expression ":" suite
               ("elif" assignment_expression ":" suite)*
               ["else" ":" suite]

It selects exactly one of the suites by evaluating the expressions one
by one until one is found to be true (see section Boolean operations
for the definition of true and false); then that suite is executed
(and no other part of the "if" statement is executed or evaluated).
If all expressions are false, the suite of the "else" clause, if
present, is executed.

Related help topics: TRUTHVALUE

You can also call help() without arguments to get Python’s Interactive Help. This will change the prompt to help>.

Listing 671 Python shell#

>>> help()

Welcome to Python 3.8's help utility!

...
help>

You can type anything you would have passed as an argument to the help() function. For example, typing print at the help> prompt is the same as typing help("print") at the Python prompt.

Listing 672 Python shell#

help> print
Help on built-in function print in module builtins:

print(...)
    print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)

    Prints the values to a stream, or to sys.stdout by default.
...

Additionally you can type:

topics - A list of general topics that you can get more information about, where each topic is in all caps. For example STRINGS, FUNCTIONS, and CALLS.
keywords - A list of reserved words that have special meanings so that they cannot be used as variable or function names. Some examples of keywords in python are for, if, and def.
symbols - A list of punctuation symbols that have special meanings. For example =, ,, and +.
modules - (NOTE: Do not run this in repl.it, as it’s buggy and might mess up your repl.) A list of available modules.

Listing 673 Python shell#

help> keywords

Here is a list of the Python keywords.  Enter any keyword to get more help.

False               class               from                or
None                continue            global              pass
True                def                 if                  raise
...

To get back to the normal Python shell type quit() or press Ctrl-D.

Glossary #

Python Shell #

prompt#

Characters displayed by the interpreter to indicate that it is ready to take input from the user.

Python shell#

Python interpreter#

a REPL for Python code

an interface where you can type in python code, and it will run it and print out the results

read-evaluate-print-loop#

REPL#

An interactive tool or environment that takes code input, evaluates or executes it, and displays the output or resulting value to the user. shells are a subset of REPLs. More advanced REPL tools and systems are comprised of an input or editor pane, and an output or results pane. Many online REPLs exist such as play.golang.org for Go, pythonfiddle.com for Python, try.ruby-lang.org for Ruby and repl.it.

tab completion#

A feature in many REPLs where pressing TAB after a few letters of a name (like a command or variable) will attempt to guess the rest.

Python Class

Python Shell

Contents

Python Shell#

Part 1. Open the Python Shell #

Part 2. Simple expressions #

Part 3. The semicolon shorthand #

Part 4. Use Up-arrow and Down-arrow to repeat history #

Part 5. Tab completion #

Part 6. Inspect thyself #

Part 7. Get help #

Glossary #

Python Shell #