Data Types in Python

Data Types in Python#

Why Should I Care About Types?#

Imagine you’re simulating a falling parachutist. You need to store:

  • The parachutist’s mass: 68.1 kg (a decimal number)

  • The drag coefficient: 12.5 kg/s (another decimal)

  • Whether the parachute is open: True or False

  • The jumper’s name: "Alex" (text)

Each of these is a different type of data. Python needs to know what type something is so it knows what operations make sense. You can multiply two numbers, but what does it mean to multiply two names?

Get the type wrong, and your code breaks. Understand types, and you’ll write code that just works.

Numbers: Integers and Floats#

The most fundamental data in numerical methods is, of course, numbers. Python has two kinds.

Integers (int)#

When you type a number without a decimal point, Python treats it as an integer, a whole number:

n_iterations = 100
temperature = -10
num_molecules = 602214085700000

Notice there are no quotes around these values. That’s important: 25 is an integer, but "25" would be text (we’ll get to that shortly).

Floating-Point (float)#

But most engineering calculations involve decimal numbers. When you include a decimal point, Python creates a float:

pressure = 101.325
molar_mass = 18.015

For very large or very small numbers, you can use scientific notation with e:

concentration = 2.5e-3  # Same as 0.0025
avogadro = 6.022e23     # 6.022 × 10²³

So now we know how to store numbers. But what about text?

Text: Strings#

Sometimes you need to store text: a compound name, a file path, a label for your plot. In Python, text is called a string (str), and it must be enclosed in quotes:

element = "Carbon"
formula = 'H2O'

You can use single quotes '...' or double quotes "...". Python treats them the same. Pick one style and be consistent.

For text that spans multiple lines, use triple quotes:

description = """This reaction produces
carbon dioxide and water
as byproducts."""

The Quotes Matter#

Here’s a common mistake. What happens if you forget the quotes?

element = Carbon

Python sees Carbon without quotes and thinks: “That must be a variable name. Let me look up its value.” But you never defined a variable called Carbon, so Python raises a NameError.

The fix is simple: always put quotes around text.

element = "Carbon"  # Correct: this is the string "Carbon"

This brings us to an interesting question: if 25 is a number but "25" is text, how do you convert between them?

Converting Between Types#

User input always comes as text. If someone types 25.5 in response to a prompt, Python gives you the string "25.5", not the number 25.5. You can’t do math with a string:

temp_str = "25.5"
new_temp = temp_str + 10  # ERROR! Can't add string and integer

To convert a string to a number, use int() or float():

temp_str = "25.5"
temp = float(temp_str)  # Now it's the number 25.5
new_temp = temp + 10    # Works! new_temp is 35.5

This matters in numerical methods. When computing the Taylor series for \(e^x\), you track both the term number (an int like 1, 2, 3…) and the running sum (a float like 2.718…).

You can also convert numbers to strings when you need to combine them with text:

pressure = 101.325
message = "Pressure is " + str(pressure) + " kPa"

A Trap: int() Truncates, It Doesn’t Round#

When you convert a float to an integer, Python chops off the decimal part. It does NOT round:

print(int(3.9))   # Output: 3 (not 4!)
print(int(-3.9))  # Output: -3 (not -4!)

If you want rounding, use the round() function instead.

True or False: Booleans#

There’s one more type you’ll use constantly: booleans. A boolean can only be one of two values: True or False.

is_exothermic = True
reaction_complete = False

You’ll use booleans in conditionals and loops:

parachute_open = False

if parachute_open:
    c = 68.0   # High drag
else:
    c = 12.5   # Low drag

Watch the Capitalization#

Python is picky about booleans:

  • True and False (capital T and F) are booleans ✓

  • true and false (lowercase) cause a NameError ✗

  • "True" (in quotes) is a string, not a boolean ✗

active = True      # ✓ Boolean
active = true      # ✗ NameError: name 'true' is not defined
active = "True"    # This "works" but it's a string, not a boolean!

How Do I Check a Variable’s Type?#

If you’re ever unsure what type a variable is, use the type() function:

x = 3.14
print(type(x))

Output:

<class 'float'>

This is useful for debugging. If your code isn’t working, check whether you have a string when you expected a number, or vice versa.

Quick Reference#

Type

Example

Notes

int

x = 42

Whole numbers, no decimals

float

x = 3.14

Decimal numbers, use e for scientific notation

str

x = "hello"

Text in quotes (single or double)

bool

x = True

Only True or False (capital T/F, no quotes)

Type Conversion:

From → To

Function

Example

str → int

int()

int("42") → 42

str → float

float()

float("3.14") → 3.14

number → str

str()

str(42) → "42"

float → int

int()

int(3.9) → 3 (truncates!)

Next Steps#

Now that you know the building blocks, continue to Variables & Operators to learn how to store and manipulate these values.

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