P2-8.1 Values, Variables, and Types¶

Section ID: P2-8.1 Version: v2026.09.15

82 and "82" look similar on screen but behave differently in calculations. 82 + 3 is 85, while "82" + 3 raises an error because numbers and strings have different types.

Values and Operations¶

Numbers 3 and 3.14, the string "AI", and truth values True and False are all values. An operation processes these values.

Adding numbers 3 and 2 gives 5; adding strings "AI" and "Book" gives AIBook. This code prints both results in order.

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print(3 + 2)
print("AI" + "Book")

The same + performs addition for numbers and concatenation for strings.

Variables and Assignment¶

Python uses = to connect a name with a value. This action is called assignment.

A rectangle of width 20 and height 45 has area 900. Here, width and height refer to the lengths, and area to their product.

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width = 20
height = 45
area = width * height
print(area)

A mathematical equation expresses equality between two sides. A Python assignment evaluates the right side, then binds the left-side name to the result. area = width * height does not maintain a continuing relationship between area and the two lengths. Changing width later does not automatically recalculate area.

The same name can be assigned a new value. Here, score first refers to 10, then to 12, and the code prints 12.

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score = 10
score = 12
print(score)

Variables are sometimes described as boxes, but in Python, names referring to values is more accurate: two names can refer to the same object.

Basic Types¶

A type is the kind of a value and determines which operations are possible. Passing a value to type() reveals its type.

This code prints the types of 3, 3.14, "AI", and True: <class 'int'>, <class 'float'>, <class 'str'>, and <class 'bool'>.

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print(type(3))
print(type(3.14))
print(type("AI"))
print(type(True))

float is often used for numbers with fractional parts. Values with a zero fractional part, such as 3.0, can also be floats.

Reassignment and Types¶

In Python, the same name can refer to values of different types. Assigning integer 82 and then string "82" to score changes the printed type from <class 'int'> to <class 'str'>.

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score = 82
print(type(score))

score = "82"
print(type(score))

The value referred to by the name has a type. Names need no advance type declaration, but operations follow type-specific rules. Adding string "82" and integer 3 raises TypeError.

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score = "82"
bonus = 3

print(score + bonus)

A string and an integer cannot be added directly with +. To add three bonus points to the score, first convert the score to a number.

Python’s Language Characteristics¶

When types are checked, how code executes, and what it is used for are different classification criteria.

These categories are not mutually exclusive. Python’s official summary describes it as a high-level, interpreted language with dynamic semantics.

Numbers and Strings¶

Adding number 1 to number 30 gives 31. Adding string "1" to string "30" prints 301.

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age_number = 30
age_text = "30"

print(age_number + 1)
print(age_text + "1")

Values read from CSV or Excel can arrive as strings depending on the reading method and data contents. Check their actual types before quantity or score calculations.

Comparisons and Booleans¶

A comparison produces a boolean value, True or False. Comparing score 92 against a minimum of 60 gives True. The code prints that result and its type, <class 'bool'>.

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score = 92
passed = score >= 60

print(passed)
print(type(passed))

>= checks whether the left value is greater than or equal to the right. Decisions such as whether a prediction score meets a threshold or data has missing values can also be expressed as booleans.

Case: Checking a String Score Against a Pass Threshold¶

Suppose we check whether student Kim’s score 82.5 meets threshold 60.0. The name is a string, the score and threshold are numbers, and the result is a boolean. The code prints Kim, 82.5, and True in order.

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student_name = "Kim"
score = 82.5
threshold = 60.0

passed = score >= threshold

print(student_name)
print(score)
print(passed)

If the score arrives from a file as string "82.5", the same comparison fails. The following raises TypeError because >= cannot compare str and float.

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score = "82.5"
threshold = 60.0

print(score >= threshold)

Use float() to convert the number written in the string into a numeric value. This code prints the types before and after conversion, <class 'str'> and <class 'float'>, then the pass result True.

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score_text = "82.5"
score = float(score_text)
threshold = 60.0

print(type(score_text))
print(type(score))
print(score >= threshold)

score_text refers to the original string and score to the converted number. Changing threshold in the last code block to 90.0 makes the result False. Matching types enables the comparison; changing the criterion changes its result.

Truth Conversion and String Contents¶

bool() does not interpret the meaning of words in a string. An empty string is false; a nonempty one is true. Applying bool() to the configuration string "False" therefore returns True.

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print(bool(""))
print(bool("False"))
print(bool("0"))
print(bool(0))

The outputs are False, True, True, and False. Numeric zero and the string "0" do not have the same truth value. For string-based settings, define the accepted spellings and conversion rules.

Numeric conversion also requires suitable contents. float("82.5") produces 82.5, but float("missing") raises ValueError. The missing-value marker None differs from zero and the string "None". Arbitrarily turning missing scores into zero loses the distinction from a real zero score, so separate missing-data handling from numeric conversion.

Checklist¶

• You can distinguish values, variables, and types.
• You can explain = as assigning a value to a name in Python.
• You can explain the basic differences among int, float, str, and bool.
• You can distinguish a numeric-looking string from an actual number.
• You can check a value’s type with type().
• You can explain why a type error calls for checking the data’s state.

• You can explain why actual types must be checked even when values look numeric on screen.

• Explain why bool("False") is True and why replacing missing None values with zero loses information.

Sources and References¶

• Python Software Foundation, What is Python? Executive Summary, Python.org, checked on 2026-07-20. Used to confirm Python's dynamic semantics, dynamic typing, and high-level built-in data structures.
• Python Software Foundation, An Informal Introduction to Python, Python 3 documentation, checked on 2026-07-20. Used to confirm how prompts, numbers, strings, and lists are introduced in interactive Python examples.
• Python Software Foundation, Built-in Types, Python 3 documentation, checked on 2026-09-15. Used to confirm basic types such as int, float, str, and bool, and differences in operations by type.
• Python Software Foundation, Data model, Python 3 documentation, checked on 2026-07-20. Used to confirm that Python objects have identity, type, and value.