CS1033
Programming Fundamentals
Semester 1 (2024 Intake)
[Jan – Jun 2025]
Lecture 2
Sanath Jayasena
Dept. of Computer Science & Engineering
Outline
Lecture 2
• Basic Python Data Types
• Objects
• Mutable vs Immutable Objects
• Input and Output
• File Read/Write
• Expressions & Operators
• Python Keywords and Built-ins
Sanath Jayasena
Dept. of Computer Science & Engineering
2
References
• CS1033 Notes, Part A
– Chapters 2, 3
– Corresponding Jupyter Notebooks
• Additional online resources
– Python Tutorials and Documentation
• https://docs.python.org/3.11/tutorial/
• https://www.tutorialspoint.com/python3/
• https://docs.python.org/3.11/
• Visualize Python execution: http://pythontutor.com/visualize.html
Sanath Jayasena
Dept. of Computer Science & Engineering
3
Outline
Lecture 2
PART 1
Sanath Jayasena
• Basic Python Data Types
• Objects
• Mutable vs Immutable Objects
• Input and Output
• File Read/Write
• Expressions & Operators
• Python Keywords and Built-ins
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Python Basic Data Types
• Three numeric types
– Integers → int
– Floating-point (real) numbers → float
– Complex numbers → complex
• Boolean type → bool
• String type → str
(sub-type of int)
… and other (compound) types
• → list , tuple , dict , set ,…
basic data structures
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Dept. of Computer Science & Engineering
5
Examples
>>> a="abc" + 'xyz'
>>> a
'abcxyz'
>>> type(a)
<class 'str'>
>>> b=97
>>> type(b)
<class 'int'>
>>> type(97.0)
<class 'float'>
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>>> d = 5 > 4
>>> d
True
>>> type(d)
<class 'bool'>
>>> e = 3 + 4j
>>> type(e)
<class 'complex'>
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Data Formats
Data Type
int
float
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Examples
-96 (decimal)
0x3bfe9 (hexadecimal)
-0o3702 (octal)
0b100101 (binary)
9.01
-9.
32.3e+18
-321.54e100
8734.098E-12
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Objects
• Objects are Python’s abstraction for data
– Data in a program represented by objects or as relations
between objects
• Every object has
– Identity : address in memory; never changes; check with id( )
function or “is” operator
– Type (class) : check with type( ) function; possible operations,
values depend on type
– Value : value of some objects can change
Sanath Jayasena
Dept. of Computer Science & Engineering
8
Object Names (Variables)
• Can associate a name with an object
• An object may have 0, 1 or more names
• A name is also called a reference, an identifier or a
variable
• Python variable naming rules:
– Must begin with a letter (a - z, A - Z) or “_”
– Then, can have letters, digits or _ only
– Variable names are case sensitive
– Keywords (reserved words) cannot be used
Sanath Jayasena
Dept. of Computer Science & Engineering
9
Outline
Lecture 2
PART 2
Sanath Jayasena
• Basic Python Data Types
• Objects
• Mutable vs Immutable Objects
• Input and Output
• File Read/Write
• Expressions & Operators
• Python Keywords and Built-ins
Dept. of Computer Science & Engineering
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Mutable vs Immutable Objects
• Value can change → mutable object
• Value cannot change → immutable object
• Mutability depends on type
– Numbers, strings, tuples, bytes, frozenset, … → immutable
– Lists, dictionaries, sets, user-defined classes, …→ mutable
– Note: we have seen only numbers and strings; we will cover
lists, tuples, dictionaries later.
Sanath Jayasena
Dept. of Computer Science & Engineering
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Immutability of Numbers
[1]>>> b = 97
[2]>>> c = b
[3]>>> c
97
[4]>>> id(b)
1638394498288
[5]>>> id(c)
1638394498288
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[6]>>> b = 98
[7]>>> b
98
[8]>>> c
97
[9]>>> id(b)
1638394498320
[10]>>> id(c)
1638394498288
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Example explained:
>>> b = 97
>>> id(b)
1638394498288
>>> c = b
>>> b = 98
>>> id(b)
1638394498320
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b
Memory
contents Memory address
97
1638394498288
c
c
b
Memory
contents Memory address
97
1638394498288
98
1638394498320
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Immutability of Strings
[1]>>> a
'abcxyz'
[2]>>> id(a)
1638435666992
[3]>>> s = a
[4]>>> id(s)
1638435666992
[5]>>> a[2]
'c'
[6]>>> a is s
True
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[7]>>> s = "pqr"
[8]>>> s
'pqr'
[9]>>> id(s)
1638435653936
[10]>>> a[4] = "W"
Traceback (most recent call last):
File "<pyshell#159>", line 1, in
<module>
a[4] = "W"
TypeError: 'str' object does not
support item assignment
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Mutability of Lists
[1]>>> x = [3,4,'mno']
[7]>>> id(y)
[2]>>> x
1638435730752
[3, 4, 'mno']
[8]>>> x[1] = "jk"
[3]>>> y = x
[9]>>> x is y
[4]>>> y
True
[3, 4, 'mno']
[10]>>> y
[5]>>> y is x
[3, 'jk', 'mno']
True
[11]>>> x
[6]>>> id(x)
[3, 'jk', 'mno']
1638435730752
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Dept. of Computer Science & Engineering
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Exercise 1
• Explain the following
>>> a = 9
>>> b = 9
>>> a is b
True
>>> a = 1000
>>> b = 1000
>>> a is b
False
Try the above with strings, first with a short string (e.g., x =
“abcd” and y = “abcd”) and then with a much longer string
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Dept. of Computer Science & Engineering
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Exercise 2
• What do the following built-in functions do?
int()
float()
complex()
str()
chr(), ord()
hex(), oct(), bin()
Sanath Jayasena
Dept. of Computer Science & Engineering
17
Outline
Lecture 2
PART 3
Sanath Jayasena
• Basic Python Data Types
• Objects
• Mutable vs Immutable Objects
• Input and Output
• File Read/Write
• Expressions & Operators
• Python Keywords and Built-ins
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Input, Output
• Output: Use the built-in function print()
• “print()” function
– Evaluates each argument (comma-separated expression) in
turn and writes resulting object to standard output (display)
– Object first converted to a string if needed
– “\n” (newline) character written at the end
• What if you don’t need “\n” at the end?
– print(objects, sep=' ', end='\n’)
• Prints objects to display, separated by sep and followed by end
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Dept. of Computer Science & Engineering
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Input, Output
• Input: Use built-in function input()to read input from
keyboard (standard input)
• input([prompt])
– Reads a line from standard input, converts it to a string
(removing trailing newline) and returns it
– If the prompt argument is present, it is written to
standard output without a trailing newline
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Dept. of Computer Science & Engineering
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Example 1
# Program-2.2
s = input("Enter your input: ")
print ("Received input is:", s)
Here is a sample run of the above program:
Enter your input: Hello Python
Received input is: Hello Python
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Dept. of Computer Science & Engineering
21
Example 2
# Program-2.3
s = input("Enter your input: ")
print("Received input is:", eval(s))
Here is a sample run of the above program:
Enter your input: [x*5 for x in [1,2,3]]
Received input is: [5, 10, 15]
Sanath Jayasena
Dept. of Computer Science & Engineering
22
Outline
Lecture 2
PART 4
Sanath Jayasena
• Basic Python Data Types
• Objects
• Mutable vs Immutable Objects
• Input and Output
• File Read/Write
• Expressions & Operators
• Python Keywords and Built-ins
Dept. of Computer Science & Engineering
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File Read/Write
• First create a file object using built-in open() function
to access the file
Many other file
• Then use the following file methods
methods available.
Python file
– file.read([size])
methods are
– file.write(str)
based on the
– file.close()
stdio library in C
• Many file attributes also available
– file.closed , file.name , file.mode , …
Sanath Jayasena
Dept. of Computer Science & Engineering
24
open() Function
• open(name [, mode [,buffering]])
– name : name of file to be opened
– mode : a string, how the file to be opened
• Example modes
– “r” for reading, “w” for writing, “a” for appending (default is “r”)
– Add “b” to mode to open in binary mode
– Modes “r+”, “w+” and “a+” open file for reading and writing
– [See: https://stackoverflow.com/questions/1466000/ for details]
Sanath Jayasena
Dept. of Computer Science & Engineering
25
Example 1
# Program-2.4
# Open a file
fo = open("pythonlearn.txt", "r")
s = fo.read()
print ("Read String is:", s)
# Close opened file
fo.close()
Sanath Jayasena
Dept. of Computer Science & Engineering
26
Example 2
# Program-2.5
# Open a file
fo = open("firstwrite.txt", "w")
fo.write("Python is fun!!\n")
# Close opened file
fo.close()
Sanath Jayasena
Dept. of Computer Science & Engineering
27
Outline
Lecture 2
PART 5
Sanath Jayasena
• Basic Python Data Types
• Objects
• Mutable vs Immutable Objects
• Input and Output
• File Read/Write
• Expressions & Operators
• Python Keywords and Built-ins
Dept. of Computer Science & Engineering
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Expressions, Operations
• Here the
• Expressions
discussion is mainly
– Arithmetic (algebraic)
on numeric types
– Logical (Boolean)
• Expressions and
• Operations
operations on lists
– Arithmetic, logical/comparison and strings will be
– Assignment
covered later
(Chapters 6, 8 in
– Bit-wise
Notes)
– Membership, Identity
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Dept. of Computer Science & Engineering
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Arithmetic Expressions, Operations
Floor (integer) division
• Examples
[1]>>> 3 + 4.5
7.5
[2]>>> (3 + 2) * 8
40
[3]>>> 5 / 2
2.5
[4]>>> 5.0 / 2
2.5
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[5]>>> 5 // 2
2
[6]>>> 5.0 // 2
2.0
Modulo
[8]>>> 7 % 3
division
1
[9]>>> 5 ** 3
125
Exponentiation 53
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Arithmetic Expressions, Operations
• Many built-in functions available
>>> abs(-4)
4
>>> abs(-3.2)
3.2
>>> min(2, -4, 6, -2)
-4
>>> max(12, 26.5, 3.5)
26.5
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Dept. of Computer Science & Engineering
31
Mixed Arithmetic (& Coercion)
• When a binary arithmetic operator has operands of
different numeric types, the operand with the “narrower”
type is “widened” (coerced) to that of the other
• Examples:
– Integers narrower than floating point
– Floating point narrower than complex numbers
– If either operand is complex (or floating point), the other is
converted to complex (or floating point)
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Dept. of Computer Science & Engineering
32
Exercise
• Do the Practice Problem 2.1 on page 18 in Notes.
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Dept. of Computer Science & Engineering
33
Boolean (Logical) Expressions
[1]>>> 2 < 3
[6]>>> 2 < 3 and 4 > 5
True
False
[2]>>> 2 == 2
[7]>>> not (5 > 3)
True
False
[3]>>> 2 != 2
[8]>>> b, c
False
(98, 97)
[4]>>> 2 >= 1
[9]>>> b > c or c >= 98
True
True
[5]>>> 5 - 1 > 2 + 1
See Sections 3.3, 3.6 in Notes
True
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Dept. of Computer Science & Engineering
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Exercise
• Do the Practice Problem 2.2 on page 19
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Dept. of Computer Science & Engineering
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Assignments
• General format of assignment statement
<variable> = <expression>
• Examples
>>> b = 4
>>> counter = b * 4
>>> counter
16
>>> 5 = b
SyntaxError: cannot assign to literal here.
Maybe you meant '==' instead of '='?
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Dept. of Computer Science & Engineering
36
Assignment Operators
Operator
=
Example
c = a + b assigns value of
a + b to c
+=
c += a same as c = c + a
-=
c -= a same as c = c - a
*=
c *= a same as c = c * a
See
Section
3.4 in
Notes
Similar others like: /=, //=, %=, **=
Sanath Jayasena
Dept. of Computer Science & Engineering
37
Exercise
1. Do the Practice Problem 2.3 on page 20
2. Explain what will be the result if we input the following to
the Python interpreter?
(i) p = q = r = 5
(ii) a, b, c = 23, 3.4, "ijk"
(iii) [x, y, z] = [1,"mn",3]
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Dept. of Computer Science & Engineering
38
Bit-wise Operators
• Assume a = 60 and b = 13
• Their binary representations and bit-wise operations
a
=
0011
1100
b
=
0000
1101
---------------------a & b =
0000
1100
a | b =
0011
1101
a ^ b =
0011
0001
~a
=
1100
0011
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Other
operators:
<< and >> (left
and right shift).
See Section 3.5
in Notes
(AND)
(OR)
(XOR)
(Complement)
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Membership Operator “in”
• Tests membership in a sequence; e.g., strings, lists, tuples
Operator
Description
Evaluates to True if it
finds an object in the
in
specified sequence;
False otherwise.
Evaluates to True if it
not in does not find an object in
the specified sequence
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Example
x in y → this
results in True if x is
a member of y.
x not in y → this
results in True if x is
not a member of y.
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Identity Operator “is”
Operator
Description
Example
is
Evaluates to True if the names
(vars) on either side of the
operator point to the same
object and False otherwise.
x is y → this
results in True if
id(x) == id(y).
Evaluates to False if the names x is not y →
(vars) on either side of the
results in True if
is not
operator point to the same
id(x) != id(y).
object and True otherwise.
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Dept. of Computer Science & Engineering
41
Operator Precedence
Operator
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**
~ , +, *, / , %, //
+,>>, <<
&
^,|
<=, < , >, >=
==, !=
= , %=, /=, //=,
-=, +=, *=, **=
is, is not
in, not in
not, or, and
Description
Exponentiation (raise to the power)
Complement, unary plus and minus
Multiply; divide, modulo, floor division
Addition and subtraction
Right and left bitwise shift
Bitwise 'AND'
Bitwise `XOR' and `OR'
Comparison operators
Equality operators
Precedence
HIGH
Assignment operators
Identity operators
Membership operators
Logical
operators
Dept. of Computer Science & Engineering
Precedence
LOW
42
Outline
Lecture 2
PART 6
Sanath Jayasena
• Basic Python Data Types
• Objects
• Mutable vs Immutable Objects
• Input and Output
• File Read/Write
• Expressions & Operators
• Python Keywords and Built-ins
Dept. of Computer Science & Engineering
43
Python Keywords
and
continue
for
lambda
try
as
def
from
nonlocal
while
assert
del
global
not
with
async
elif
if
or
yield
await
else
import
pass
False
break
except
in
raise
True
class
finally
is
return
None
Sanath Jayasena
Dept. of Computer Science & Engineering
44
Python Built-in Functions
For complete list, see https://docs.python.org/3/library/functions.html
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Dept. of Computer Science & Engineering
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Python Built-in Functions
For complete list, see https://docs.python.org/3/library/functions.html
Sanath Jayasena
Dept. of Computer Science & Engineering
46
Python Built-in Constants
• False, True
– False and true values of bool type
• None
– Used to represent the absence of a value
>>> d = 5
>>> d
5
>>> d = None
>>> d
(nothing displayed)
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>>> if (d == None):
print("Hello")
Hello
>>>
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47
Conclusion
• We discussed
– Data types, Objects, Mutability
– Input, Output
– File Read/Write
– Expressions, Operations
– Operators
– Python keywords, Built-ins
Sanath Jayasena
Dept. of Computer Science & Engineering
48