Convert carefully
Input is text. Convert only after deciding which values are valid.
Functions & Files · Unit 4
Built-in Functions
Conversion, math and string functions
Unit 4 · Python’s ready-made toolkit
Choose a small tool. Understand its contract.
A built-in function is available without importing a module. It accepts inputs (arguments), follows a defined rule, and returns a result. String operations such as strip() are methods called on a string; len() is a built-in function that accepts a string. Select an entry to load an editable example into the live runner.
Aggregate honestly
sum(), min(), max(), and len() answer different questions.
Strings do not mutate
Methods return a new string. Save the result to keep the transformation.
Function explorer
Browse all 45 names listed in the course plan: 12 conversion, 7 numeric, and 26 string operations.
Conversion · built-in function
int("42")
Convert a valid numeric value to an integer.
Expected result
42 · intRemember: int(3.9) truncates toward zero; it does not round.
Try int()
Starting up Python…value = int("42")
print(value)
print(type(value).__name__)The editor is ready while Python loads in the background.
Output
Your program's output will appear here.
Common Python mistakes
NameError: name 'mark' is not defined
NameError: A name was used before Python could find a variable or function with that name.
IndexError: list index out of range
IndexError: The requested position is outside the items in this list or sequence.
KeyError: 'email'
KeyError: That key does not exist in this dictionary.
TypeError: can only concatenate str (not "int") to str
TypeError: This operation does not work with the kind of value it received.
ZeroDivisionError: division by zero
ZeroDivisionError: A number cannot be divided by zero.
SyntaxError: invalid syntax
SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.
IndentationError: expected an indented block
IndentationError: The spaces at the start of a line do not match Python's block structure.
Worked patterns
Combine tools to answer a real question
Change the data and predict the output before running each example.
Validate, then convert
Conversion may fail for invalid text. Production code should validate or handle ValueError.
Conversion worked example
Starting up Python…raw_marks = "91" marks = int(raw_marks) print(marks + 4) print(type(marks).__name__)
The editor is ready while Python loads in the background.
Output
Your program's output will appear here.
Common Python mistakes
NameError: name 'mark' is not defined
NameError: A name was used before Python could find a variable or function with that name.
IndexError: list index out of range
IndexError: The requested position is outside the items in this list or sequence.
KeyError: 'email'
KeyError: That key does not exist in this dictionary.
TypeError: can only concatenate str (not "int") to str
TypeError: This operation does not work with the kind of value it received.
ZeroDivisionError: division by zero
ZeroDivisionError: A number cannot be divided by zero.
SyntaxError: invalid syntax
SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.
IndentationError: expected an indented block
IndentationError: The spaces at the start of a line do not match Python's block structure.
Summarize class marks
sum() gives a total, len() gives a count, and round() creates a useful display value.
Numbers worked example
Starting up Python…marks = [82, 91, 76]
average = sum(marks) / len(marks)
print("Total:", sum(marks))
print("Highest:", max(marks))
print("Average:", round(average, 1))The editor is ready while Python loads in the background.
Output
Your program's output will appear here.
Common Python mistakes
NameError: name 'mark' is not defined
NameError: A name was used before Python could find a variable or function with that name.
IndexError: list index out of range
IndexError: The requested position is outside the items in this list or sequence.
KeyError: 'email'
KeyError: That key does not exist in this dictionary.
TypeError: can only concatenate str (not "int") to str
TypeError: This operation does not work with the kind of value it received.
ZeroDivisionError: division by zero
ZeroDivisionError: A number cannot be divided by zero.
SyntaxError: invalid syntax
SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.
IndentationError: expected an indented block
IndentationError: The spaces at the start of a line do not match Python's block structure.
Normalize a name
strip() removes edge spaces, title() changes capitalization, and startswith() checks the cleaned result.
Strings worked example
Starting up Python…student = " mira patel "
clean_name = student.strip().title()
print(clean_name)
print("Length:", len(clean_name))
print("Starts with Mira:", clean_name.startswith("Mira"))The editor is ready while Python loads in the background.
Output
Your program's output will appear here.
Common Python mistakes
NameError: name 'mark' is not defined
NameError: A name was used before Python could find a variable or function with that name.
IndexError: list index out of range
IndexError: The requested position is outside the items in this list or sequence.
KeyError: 'email'
KeyError: That key does not exist in this dictionary.
TypeError: can only concatenate str (not "int") to str
TypeError: This operation does not work with the kind of value it received.
ZeroDivisionError: division by zero
ZeroDivisionError: A number cannot be divided by zero.
SyntaxError: invalid syntax
SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.
IndentationError: expected an indented block
IndentationError: The spaces at the start of a line do not match Python's block structure.
Common mistakes and good habits
TypeError
sum(["82", "91"])These are strings, not numbers. Convert validated numeric text before arithmetic.
ValueError
int("nine")Python cannot parse this as an integer. Validate input and explain the expected format.
Result not saved
name.strip() # result ignoredStrings are immutable. Assign the result: name = name.strip().
Boundary surprise
"room 2".isalpha()False: isalpha requires every character to be a letter.
Check your understanding
1. What does int(3.9) produce?
2. Which expression counts items in marks?
3. What is " Mira ".strip()?
4. What does "banana".find("z") return?
Choose functions by asking three questions
What type is the data now? What result do I need? What should happen for empty or invalid data? For example, guard an average against an empty list before dividing by len(marks). Knowing edge cases is part of using a function well.