Python Tokens, Identifiers, Variables, Data Types and Mutability Class 11

Understand Python Tokens, Identifiers, Variables, Data Types and Mutability

a = 10
b = a
b = b + 5
print(a, b)

x = [1, 2]
y = x
y.append(3)
print(x)

Python Tokens, Identifiers, Variables, Data Types and Mutability: key facts

Identifier rules

• •Start with a letter or underscore
• •Then letters, digits, underscore
• •No spaces or symbols
• •Case-sensitive
• •Not a keyword

Valid: _x, Total2. Invalid: 2nd, my-var, if.

Immutable types

• •int
• •float
• •bool
• •str
• •tuple

A new object is made when changed.

Mutable types

• •list
• •dict
• •set

Changed in place; aliases see the change.

Useful functions

type	object
id	object
print	value

type gives the class of the object.

How to answer Python Tokens, Identifiers, Variables, Data Types and Mutability questions

1. 1Draw each name pointing to its object.
2. 2For immutable objects, an update makes a new object and rebinds the name.
3. 3For mutable objects, methods like append change the shared object.
4. 4For identifier questions, test each rule in order: first character, other characters, keyword.
5. 5Write the output with the exact type names, such as <class 'int'>.

Python Tokens, Identifiers, Variables, Data Types and Mutability in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Python Tokens, Identifiers, Variables, Data Types and Mutability

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Strings and tuples are locked

  Immutable: int, float, bool, str, tuple.

  Use it when: Mutability.

• Start with a letter, no hyphen

  Identifiers cannot begin with a digit or contain hyphens.

  Use it when: Valid names.

Python Tokens, Identifiers, Variables, Data Types and Mutability: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

a = 10
b = a
b = b + 5
print(a, b)
x = [1, 2]
y = x
y.append(3)
print(x)

print(type(5), type(5.0), type("5"), type(True), type(None))
a, b = 3, 4
a, b = b, a
print(a, b)

a = [1, 2, 3]
b = a
c = a[:]
b.append(4)
c.append(5)
print(a, b, c)

Where students lose marks in Python Tokens, Identifiers, Variables, Data Types and Mutability

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying lists are immutable

  Confusing lists and tuples.

  Wrong working

  A list is an immutable type.

  Right working

  A list is mutable; a tuple is immutable.

  ✓Do this: List: changeable; tuple: fixed.

  Usually costs: 1 mark

• ✗Mistake: Believing b = a copies a list

  Assignment copies the reference.

  Wrong working

  b is a new list after b = a.

  Right working

  b and a refer to the same list.

  ✓Do this: Use a[:] or copy() for a copy.

  Usually costs: 1 mark

• ✗Mistake: Starting an identifier with a digit

  Overlooking the first-character rule.

  Wrong working

  1total is valid.

  Right working

  1total is invalid.

  ✓Do this: First character: a letter or underscore.

  Usually costs: 1 mark

Practice questions on Python Tokens, Identifiers, Variables, Data Types and Mutability

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which of the following is mutable?
   (a) int
   (b) str
   (c) tuple
   (d) list

   Hint

   Changeable in place.

   Answer

   (d) list

2. Q2 · 3 marks · Short answerHarder board level

   What is the output?

   x = 5
   y = x
   x = x * 2
   print(x, y)
   s = "ab"
   t = s
   s = s + "c"
   print(s, t)

   (3 marks)

   Hint

   Immutable objects are rebound.

   Answer

   x = 5; y = 5; x becomes 10, y stays 5. Output: 10 5. s = "abc" is a new string; t stays "ab". Output: abc ab.

3. Q3 · 3 marks · Short answerHarder board level

   Write the data type of: 7, 7.0, "7", 7 > 3, [7], (7,), {7: 7}, None. (3 marks)

   Hint

   Think of the literal.

   Answer

   int, float, str, bool, list, tuple, dict, NoneType.

4. Q4 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): After a = [1, 2]; b = a; b[0] = 9, the value of a[0] is 9.
   Reason (R): a and b refer to the same list object.
   (a) Both A and R are true, and R is the correct explanation of A.
   (b) Both A and R are true, but R is not the correct explanation of A.
   (c) A is true, but R is false.
   (d) A is false, but R is true.

   Hint

   Alias.

   Answer

   (a) Both A and R are true, and R is the correct explanation of A.

5. Q5 · 5 marks · Short answerHOTS / competency

   Find the output and explain each line:

   a = [1, 2]
   b = a
   a = a + [3]
   print(a, b)

   How does it differ from a.append(3)? (5 marks)

   Hint

   a + [3] makes a new list.

   Answer

   a + [3] creates a new list [1, 2, 3] and rebinds a; b still refers to the old list [1, 2]. Output: [1, 2, 3] [1, 2]. With a.append(3) the same list is changed, so b would also show [1, 2, 3].

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Can an identifier start with a digit?Show answer

   No.

2. 2.Is a list mutable?Show answer

   Yes.

3. 3.Is a string mutable?Show answer

   No.

4. 4.Function that gives the type?Show answer

   type().

5. 5.What does a, b = b, a do?Show answer

   Swaps the values.

Python Tokens, Identifiers, Variables, Data Types and Mutability: frequently asked questions