Ana Sayfa / Python / Getting started with Python Language

Getting started with Python Language

Getting Started

Python is a widely used high-level programming language for general-purpose programming, created by Guido van Rossum and first released in 1991. Python features a dynamic type system and automatic memory management and supports multiple programming paradigms, including object-oriented, imperative, functional programming, and procedural styles. It has a large and comprehensive standard library.

Two major versions of Python are currently in active use:

  • Python 3.x is the current version and is under active development.
  • Python 2.x is the legacy version and will receive only security updates until 2020. No new features will be implemented. Note that many projects still use Python 2, although migrating to Python 3 is getting easier.

You can download and install either version of Python here. See Python 3 vs. Python 2 for a comparison between them. In addition, some third-parties offer re-packaged versions of Python that add commonly used libraries and other features to ease setup for common use cases, such as math, data analysis or scientific use. See the list at the official site.

Verify if Python is installed

To confirm that Python was installed correctly, you can verify that by running the following command in yourfavorite terminal (If you are using Windows OS, you need to add path of python to the environment variable before using it in command prompt):

$ python –version
Python 3.x Version ≥ 3.0

If you have Python 3 installed, and it is your default version (see Troubleshooting for more details) you should see.

something like this:

$ python –version
Python 3.6.0
Python 2.x Version ≤ 2.7

If you have Python 2 installed, and it is your default version (see Troubleshooting for more details) you should see something like this:

$ python –version
Python 2.7.13

If you have installed Python 3, but $ python –version outputs a Python 2 version, you also have Python 2
installed. This is often the case on MacOS, and many Linux distributions. Use $ python3 instead to explicitly use the Python 3 interpreter.

Hello, World in Python using IDLE

IDLE is a simple editor for Python, that comes bundled with Python.

How to create Hello, World program in IDLE

  • Open IDLE on your system of choice.
    • In older versions of Windows, it can be found at All Programs under the Windows menu.
    • In Windows 8+, search for IDLE or find it in the apps that are present in your system.
    • On Unix-based (including Mac) systems you can open it from the shell by typing $ idle
  • It will open a shell with options along the top.

In the shell, there is a prompt of three right angle brackets:


Now write the following code in the prompt:

>>> print (“Hello World”)

Hit Enter .

>>> print (“Hello World”)
Hello, World

Hello World Python file

Create a new file that contains the following line:

Python 3.x Version ≥ 3.0
print(‘Hello, World’)
Python 2.x Version ≥ 2.6

You can use the Python 3 print function in Python 2 with the following import statement:

from future import print_function

Python 2 has a number of functionalities that can be optionally imported from Python 3 using the future

module, as discussed here.

Python 2.x Version ≤ 2.7

If using Python 2, you may also type the line below. Note that this is not valid in Python 3 and thus not
recommended because it reduces cross-version code compatibility.

print ‘Hello, World

In your terminal, navigate to the directory containing the file

Type python, then hit the Enter key.

$ python
Hello, World

You should see Hello, World printed to the console.

You can also substitute with the path to your file. For example, if you have the file in your home directory and your user is “user” on Linux, you can type python /home/user/

Launch an interactive Python shell

By executing (running) the python command in your terminal, you are presented with an interactive Python shell. This is also known as the Python Interpreter or a REPL (for ‘Read Evaluate Print Loop’).

$ python
Python 2.7.12 (default, Jun 28 2016, 08:46:01)
[GCC 6.1.1 20160602] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print 'Hello, World'
Hello, World

If you want to run Python 3 from your terminal, execute the command python3.

$ python3
Python 3.6.0 (default, Jan 13 2017, 00:00:00)
[GCC 6.1.1 20160602] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print('Hello, World')
Hello, World

Alternatively, start the interactive prompt and load file with python -i .

In command line, run:

$ python -i
"Hello World"

There are multiple ways to close the Python shell:

>>> exit()
>>> quit()

Alternatively, CTRL + D will close the shell and put you back on your terminal’s command line.

If you want to cancel a command you’re in the middle of typing and get back to a clean command prompt, while staying inside the Interpreter shell, use CTRL + C .

Try an interactive Python shell online.

Other Online Shells

Various websites provide online access to Python shells.

Online shells may be useful for the following purposes:

  • Run a small code snippet from a machine which lacks python installation(smartphones, tablets etc).
  • Learn or teach basic Python.
  • Solve online judge problems.


Disclaimer: documentation author(s) are not affiliated with any resources listed below.

Run commands as a string

Python can be passed arbitrary code as a string in the shell:

$ python -c 'print("Hello, World")'
Hello, World

This can be useful when concatenating the results of scripts together in the shell.

Shells and Beyond

Package Management – The PyPA recommended tool for installing Python packages is PIP. To install, on your command line execute pip install . For instance, pip install numpy. (Note: On windows
you must add pip to your PATH environment variables. To avoid this, use python -m pip install )

Shells – So far, we have discussed different ways to run code using Python’s native interactive shell. Shells use Python’s interpretive power for experimenting with code real-time. Alternative shells include IDLE – a pre-bundled GUI, IPython – known for extending the interactive experience, etc.

Programs – For long-term storage you can save content to .py files and edit/execute them as scripts or programs.

with external tools e.g. shell, IDEs (such as PyCharm), Jupyter notebooks, etc. Intermediate users may use these tools; however, the methods discussed here are sufficient for getting started.

Python tutor allows you to step through Python code so you can visualize how the program will flow, and helps you to understand where your program went wrong.

PEP8 defines guidelines for formatting Python code. Formatting code well is important so you can quickly read what the code does.

Creating variables and assigning values

To create a variable in Python, all you need to do is specify the variable name, and then assign a value to it.

<variable name> = <value>

Python uses = to assign values to variables. There’s no need to declare a variable in advance (or to assign a data type to it), assigning a value to a variable itself declares and initializes the variable with that value. There’s no way to declare a variable without assigning it an initial value.

# Integer
a = 2
# Output: 2

# Integer 
b = 9223372036854775807
# Output: 9223372036854775807

# Floating point
pi = 3.14
# Output: 3.14

# String
c = 'A'
# Output: A

# String 
name = 'John Doe'
# Output: John Doe

# Boolean 
q = True
# Output: True

# Empty value or null data type
x = None
# Output: None

Variable assignment works from left to right. So the following will give you an syntax error.

0 = x
=> Output: SyntaxError: can't assign to literal

You can not use python’s keywords as a valid variable name. You can see the list of keyword by:

import keyword

Rules for variable naming:

  1. Variables names must start with a letter or an underscore.
x = True # valid
 _y = True # valid
 9x = False # starts with numeral
=> SyntaxError: invalid syntax 
 $y = False # starts with symbol
=> SyntaxError: invalid syntax

2. The remainder of your variable name may consist of letters, numbers and underscores.

has_0_in_it = "Still Valid"

3. Names are case sensitive.

x = 9 
y = X*5 
=>NameError: name 'X' is not defined

Even though there’s no need to specify a data type when declaring a variable in Python, while allocating the necessary area in memory for the variable, the Python interpreter automatically picks the most suitable built-in type for it:

a = 2
# Output: <type 'int'>

b = 9223372036854775807
# Output: <type 'int'>

pi = 3.14
# Output: <type 'float'>
c = 'A'

# Output: <type 'str'>

name = 'John Doe'
# Output: <type 'str'>

q = True
# Output: <type 'bool'>

x = None

Now you know the basics of assignment, let’s get this subtlety about assignment in python out of the way.

When you use = to do an assignment operation, what’s on the left of = is a name for the object on the right. Finally, what = does is assign the reference of the object on the right to the name on the left.

That is:

a_name = an_object # "a_name" is now a name for the reference to the object "an_object"

So, from many assignment examples above, if we pick pi = 3.14, then pi is a name (not the name, since an object can have multiple names) for the object 3.14. If you don’t understand something below, come back to this point and read this again! Also, you can take a look at this for a better understanding.

You can assign multiple values to multiple variables in one line. Note that there must be the same number of arguments on the right and left sides of the = operator:

a, b, c = 1, 2, 3
print(a, b, c)
# Output: 1 2 3
a, b, c = 1, 2
=> Traceback (most recent call last):
=> File "", line N, in <module>
=> a, b, c = 1, 2
=> ValueError: need more than 2 values to unpack
a, b = 1, 2, 3
=> Traceback (most recent call last):
=> File "", line N, in <module>
=> a, b = 1, 2, 3
=> ValueError: too many values to unpack

The error in last example can be obviated by assigning remaining values to equal number of arbitrary variables. This dummy variable can have any name, but it is conventional to use the underscore (_) for assigning unwanted values:

a, b, _ = 1, 2, 3
print(a, b)
# Output: 1, 2
Note that the number of _ and number of remaining values must be equal. Otherwise 'too many values to unpack
error' is thrown as above:
a, b, _ = 1,2,3,4
=>Traceback (most recent call last):
=>File "", line N, in <module>
=>a, b, _ = 1,2,3,4
=>ValueError: too many values to unpack (expected 3)

You can also assign a single value to several variables simultaneously.

a = b = c = 1
print(a, b, c)
# Output: 1 1 1

When using such cascading assignment, it is important to note that all three variables a, b and c refer to the same object in memory, an int object with the value of 1. In other words, a, b and c are three different names given to the same int object. Assigning a different object to one of them afterwards doesn’t change the others, just as expected:

a = b = c = 1 # all three names a, b and c refer to same int object with value 1
print(a, b, c)
# Output: 1 1 1
b = 2 # b now refers to another int object, one with a value of 2
print(a, b, c)
# Output: 1 2 1 # so output is as expected.

The above is also true for mutable types (like list, dict, etc.) just as it is true for immutable types (like int, string, tuple, etc.):

x = y = [7, 8, 9] # x and y refer to the same list object just created, [7, 8, 9]
x = [13, 8, 9] # x now refers to a different list object just created, [13, 8, 9]
print(y) # y still refers to the list it was first assigned
# Output: [7, 8, 9]

So far so good. Things are a bit different when it comes to modifying the object (in contrast to assigning the name to a different object, which we did above) when the cascading assignment is used for mutable types. Take a look below, and you will see it first hand:

x = y = [7, 8, 9] # x and y are two different names for the same list object just created, [7,
8, 9]
x[0] = 13 # we are updating the value of the list [7, 8, 9] through one of its names, x
in this case
print(y) # printing the value of the list using its other name
# Output: [13, 8, 9] # hence, naturally the change is reflected

Nested lists are also valid in python. This means that a list can contain another list as an element

x = [1, 2, [3, 4, 5], 6, 7] # this is nested list
print x[2]
# Output: [3, 4, 5]
print x[2][1]
# Output: 4

Lastly, variables in Python do not have to stay the same type as which they were first defined — you can simply use = to assign a new value to a variable, even if that value is of a different type.

a = 2
# Output: 2
a = "New value"
# Output: New value

If this bothers you, think about the fact that what’s on the left of = is just a name for an object. First you call the int object with value 2 a, then you change your mind and decide to give the name a to a string object, having value ‘New value’. Simple, right?

Block Indentation

Python uses indentation to define control and loop constructs. This contributes to Python’s readability, however, it requires the programmer to pay close attention to the use of whitespace. Thus, editor miscalibration could result in code that behaves in unexpected ways.

Python uses the colon symbol (:) and indentation for showing where blocks of code begin and end (If you come from another language, do not confuse this with somehow being related to the ternary operator). That is, blocks in Python, such as functions, loops, if clauses and other constructs, have no ending identifiers. All blocks start with a colon and then contain the indented lines below it.

For example:

def my_function(): # This is a function definition. Note the colon (:)
 a = 2 # This line belongs to the function because it's indented
 return a # This line also belongs to the same function
print(my_function()) # This line is OUTSIDE the function block


def my_function(): # This is a function definition. Note the colon (:)
 a = 2 # This line belongs to the function because it's indented
 return a # This line also belongs to the same function
print(my_function()) # This line is OUTSIDE the function block


if a > b: # If block starts here
 print(a) # This is part of the if block
else: # else must be at the same level as if
 print(b) # This line is part of the else block

Blocks that contain exactly one single-line statement may be put on the same line, though this form is generally not considered good style:

if a > b: print(a)
else: print(b)

Attempting to do this with more than a single statement will not work:

if x > y: y = x
 print(y) # IndentationError: unexpected indent
if x > y: while y != z: y -= 1 # SyntaxError: invalid syntax

An empty block causes an IndentationError. Use pass (a command that does nothing) when you have a block with no content:

def will_be_implemented_later():

Spaces vs. Tabs

In short: always use 4 spaces for indentation

Using tabs exclusively is possible but PEP 8, the style guide for Python code, states that spaces are preferred.

Python 3.x Version ≥ 3.0
Python 3 disallows mixing the use of tabs and spaces for indentation. In such case a compile-time error is
generated: Inconsistent use of tabs and spaces in indentation and the program will not run.

Python 2.x Version ≤ 2.7
Python 2 allows mixing tabs and spaces in indentation; this is strongly discouraged. The tab character completesthe previous indentation to be a multiple of 8 spaces. Since it is common that editors are configured to show tabs as multiple of 4 spaces, this can cause subtle bugs.

When invoking the Python 2 command line interpreter with the -t option, it issues warnings about code
that illegally mixes tabs and spaces. When using -tt these warnings become errors. These options are
highly recommended!

Many editors have “tabs to spaces” configuration. When configuring the editor, one should differentiate between the tab character (‘\t’) and the Tab key.

  • The tab character should be configured to show 8 spaces, to match the language semantics – at least in cases when (accidental) mixed indentation is possible. Editors can also automatically convert the tab character to spaces.
  • However, it might be helpful to configure the editor so that pressing the Tab key will insert 4 spaces, instead of inserting a tab character.

Python source code written with a mix of tabs and spaces, or with non-standard number of indentation spaces can be made pep8-conformant using autopep8. (A less powerful alternative comes with most Python installations:


Built-in Types

bool: A boolean value of either True or False. Logical operations like and, or, not can be performed on booleans.

x or y # if x is False then y otherwise x
x and y # if x is False then x otherwise y
not x # if x is True then False, otherwise True

In Python 2.x and in Python 3.x, a boolean is also an int. The bool type is a subclass of the int type and True and False are its only instances:

issubclass(bool, int) # True

isinstance(True, bool) # True
isinstance(False, bool) # True

If boolean values are used in arithmetic operations, their integer values (1 and 0 for True and False) will be used to return an integer result:

True + False == 1 # 1 + 0 == 1
True * True == 1 # 1 * 1 == 1


a = 2
b = 100
c = 123456789
d = 38563846326424324

Integers in Python are of arbitrary sizes.
Note: in older versions of Python, a long type was available and this was distinct from int. The two have
been unified.

  • float: Floating point number; precision depends on the implementation and system architecture, for CPython the float datatype corresponds to a C double.
a = 2.0
b = 100.e0
c = 123456789.e1
  • complex: Complex numbers
a = 2 + 1j
b = 100 + 10j

The <, <=, > and >= operators will raise a TypeError exception when any operand is a complex number.

Python 3.x Version ≥ 3.0

  • str: a unicode string. The type of ‘hello’
  • bytes: a byte string. The type of b’hello’

Python 2.x Version ≤ 2.7

  • str: a byte string. The type of ‘hello’
  • bytes: synonym for str
  • unicode: a unicode string. The type of u’hello’

Sequences and collections

Python differentiates between ordered sequences and unordered collections (such as set and dict).

  • strings (str, bytes, unicode) are sequences
  • reversed: A reversed order of str with reversed function
a = reversed('hello')
  • tuple: An ordered collection of n values of any type (n >= 0).
a = (1, 2, 3)
b = ('a', 1, 'python', (1, 2))
b[2] = 'something else' # returns a TypeError

Supports indexing; immutable; hashable if all its members are hashable

  • list: An ordered collection of n values (n >= 0
a = [1, 2, 3]
b = ['a', 1, 'python', (1, 2), [1, 2]]
b[2] = 'something else' # allowed

Not hashable; mutable.

  • set: An unordered collection of unique values. Items must be hashable.
a = {1, 2, 'a'}
  • dict: An unordered collection of unique key-value pairs; keys must be hashable.
a = {1: 'one',
 2: 'two'}

b = {'a': [1, 2, 3],
 'b': 'a string'}

An object is hashable if it has a hash value which never changes during its lifetime (it needs a hash()
method), and can be compared to other objects (it needs an eq() method). Hashable objects which
compare equality must have the same hash value.

Built-in constants

In conjunction with the built-in datatypes there are a small number of built-in constants in the built-in namespace:

  • True: The true value of the built-in type bool
  • False: The false value of the built-in type bool
  • None: A singleton object used to signal that a value is absent.
  • Ellipsis or …: used in core Python3+ anywhere and limited usage in Python2.7+ as part of array notation.numpy and related packages use this as a ‘include everything’ reference in arrays.
  • NotImplemented: a singleton used to indicate to Python that a special method doesn’t support the specificarguments, and Python will try alternatives if available
a = None # No value will be assigned. Any valid datatype can be assigned later

Python 3.x Version ≥ 3.0

None doesn’t have any natural ordering. Using ordering comparison operators (<, <=, >=, >) isn’t supported anymore and will raise a TypeError

Python 2.x Version ≤ 2.7

None is always less than any number (None < -32 evaluates to True).

Testing the type of variables

In python, we can check the datatype of an object using the built-in function type.

a = '123'
# Out: <class 'str'>
b = 123
# Out: <class 'int'>

In conditional statements it is possible to test the datatype with isinstance. However, it is usually not encouraged to rely on the type of the variable.

i = 7
if isinstance(i, int):
 i += 1
elif isinstance(i, str):
 i = int(i)
 i += 1

For information on the differences between type() and isinstance() read: Differences between isinstance and type in Python

To test if something is of NoneType:

x = None
if x is None:
 print('Not a surprise, I just defined x as None.')

Converting between datatypes

You can perform explicit datatype conversion.
For example, ‘123’ is of str type and it can be converted to integer using int function

a = '123'
b = int(a)

Converting from a float string such as ‘123.456’ can be done using float function.

a = '123.456'
b = float(a)
c = int(a) # ValueError: invalid literal for int() with base 10: '123.456'
d = int(b) # 123

You can also convert sequence or collection types

a = 'hello'
list(a) # ['h', 'e', 'l', 'l', 'o']
set(a) # {'o', 'e', 'l', 'h'}
tuple(a) # ('h', 'e', 'l', 'l', 'o')

Explicit string type at definition of literals

With one letter labels just in front of the quotes you can tell what type of string you want to define.

  • b’foo bar’: results bytes in Python 3, str in Python 2
  • u’foo bar’: results str in Python 3, unicode in Python 2
  • ‘foo bar’: results str
  • r’foo bar’: results so called raw string, where escaping special characters is not necessary, everything is taken verbatim as you typed
normal = 'foo\nbar' # foo
 # bar
escaped = 'foo\\nbar' # foo\nbar 
raw = r'foo\nbar' # foo\nbar

Mutable and Immutable Data Types

An object is called mutable if it can be changed. For example, when you pass a list to some function, the list can be changed:

def f(m):
 m.append(3) # adds a number to the list. This is a mutation.

x = [1, 2]
x == [1, 2] # False now, since an item was added to the list

An object is called immutable if it cannot be changed in any way. For example, integers are immutable, since there’s no way to change them:

An object is called immutable if it cannot be changed in any way. For example, integers are immutable, since there's
no way to change them:

Note that variables themselves are mutable, so we can reassign the variable x, but this does not change the object that x had previously pointed to. It only made x point to a new object.

Data types whose instances are mutable are called mutable data types, and similarly for immutable objects and datatypes.

Examples of immutable Data Types:

  • int, long, float, complex
  • str
  • bytes
  • tuple
  • frozenset

Examples of mutable Data Types:

  • bytearray
  • list
  • set
  • dict

Collection Types

There are a number of collection types in Python. While types such as int and str hold a single value, collection types hold multiple values.


The list type is probably the most commonly used collection type in Python. Despite its name, a list is more like an array in other languages, mostly JavaScript. In Python, a list is merely an ordered collection of valid Python values. A list can be created by enclosing values, separated by commas, in square brackets:

int_list = [1, 2, 3]
string_list = ['abc', 'defghi']

A list can be empty:

The elements of a list are not restricted to a single data type, which makes sense given that Python is a dynamic language:

mixed_list = [1, 'abc', True, 2.34, None]

A list can contain another list as its element:

nested_list = [['a', 'b', 'c'], [1, 2, 3]]

The elements of a list can be accessed via an index, or numeric representation of their position. Lists in Python are zero-indexed meaning that the first element in the list is at index 0, the second element is at index 1 and so on:

names = ['Alice', 'Bob', 'Craig', 'Diana', 'Eric']
print(names[0]) # Alice
print(names[2]) # Craig

Indices can also be negative which means counting from the end of the list (-1 being the index of the last element). So, using the list from the above example:

IDLE – Python GUI

IDLE is Python’s Integrated Development and Learning Environment and is an alternative to the command line. As the name may imply, IDLE is very useful for developing new code or learning python. On Windows this comes with the Python interpreter, but in other operating systems you may need to install it through your package manager.

The main purposes of IDLE are:

  • Multi-window text editor with syntax highlighting, autocompletion, and smart indent
  • Python shell with syntax highlighting
  • Integrated debugger with stepping, persistent breakpoints, and call stack visibility
  • Automatic indentation (useful for beginners learning about Python’s indentation)
  • Saving the Python program as .py files and run them and edit them later at any them using IDLE.

User Input

To get input from the user, use the input function (note: in Python 2.x, the function is called raw_input instead, although Python 2.x has its own version of input that is completely different):

name = raw_input("What is your name? ")
# Out: What is your name? _

Security Remark Do not use input() in Python2 – the entered text will be evaluated as if it were a
Python expression (equivalent to eval(input()) in Python3), which might easily become a vulnerability.
See this article for further information on the risks of using this function.

Built in Modules and Functions

A module is a file containing Python definitions and statements. Function is a piece of code which execute some logic.

>>> pow(2,3) #8

To check the built in function in python we can use dir(). If called without an argument, return the names in the current scope. Else, return an alphabetized list of names comprising (some of) the attribute of the given object, and of attributes reachable from it.

Creating a module

A module is an importable file containing definitions and statements.

A module can be created by creating a .py file.

def say_hello():

For modules that you have made, they will need to be in the same directory as the file that you are importing them into. (However, you can also put them into the Python lib directory with the pre-included modules, but should be avoided if possible.)

Installation of Python 2.7.x and 3.x

Note: Following instructions are written for Python 2.7 (unless specified): instructions for Python 3.x are


First, download the latest version of Python 2.7 from the official Website Version is provided as an MSI package. To install it manually, just double-click the file.

By default, Python installs to a directory:

Warning: installation does not automatically modify the PATH environment variable.

Assuming that your Python installation is in C:\Python27, add this to your PATH:

Now to check if Python installation is valid write in cmd:
python –version

String function – str() and repr()

There are two functions that can be used to obtain a readable representation of an object.

repr(x) calls x.repr(): a representation of x. eval will usually convert the result of this function back to the
original object.

str(x) calls x.str(): a human-readable string that describes the object. This may elide some technical detail.


For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(). Otherwise, the representation is a string enclosed in angle brackets that contains the name of the type of the object along with additional information. This often includes the name and address of the object.
For strings, this returns the string itself. The difference between this and repr(object) is that str(object) does not always attempt to return a string that is acceptable to eval(). Rather, its goal is to return a printable or ‘human

Installing external modules using pip

pip is your friend when you need to install any package from the plethora of choices available at the python package index (PyPI). pip is already installed if you’re using Python 2 >= 2.7.9 or Python 3 >= 3.4 downloaded from For computers running Linux or another *nix with a native package manager, pip must often be manually installed.

Help Utility

Python has several functions built into the interpreter. If you want to get information of keywords, built-in
functions, modules or topics open a Python console and enter

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