Data Types and Operations¶

Learning Outcomes¶

Understand basic Python data types and operations
Store Python objects in variables
Write simple Python code to do arithmetic
Perform basic operations with lists and dictionaries!

This slide deck is a notebook!

More resources linked in the class notes.

Numbers¶

Python supports numbers. We can write integers:

In [1]:

Out[1]:

Floating-point numbers:

In [2]:

3.5

Out[2]:

3.5

Numbers with scientific notation:

In [3]:

6.02e23

Out[3]:

6.02e+23

Arithmetic Operations¶

The usual arithmetic operations (+, -, /, *) work as expected from math and other programming languages:

In [4]:

5 + 2

Out[4]:

In [5]:

2 + 3 * 6

Out[5]:

As do parentheses:

In [6]:

(2 + 3) * 6

Out[6]:

Additional Operations¶

The ** operator computes powers:

In [7]:

2 ** 5

Out[7]:

The math and numpy modules contain many math functions:

In [8]:

import numpy as np
np.log(20)

Out[8]:

2.995732273553991

Variables¶

A variable lets us give a name to a value:

In [9]:

x = 7

Just assign - no declaration necessary.

Then we can use them:

In [12]:

x + 5

Out[12]:

Let's seen an example of changing the variable:

In [11]:

x = 2

Strings¶

Python strings are written in double (") or single (') quotes — there is no difference.

In [13]:

"Hello, world"

Out[13]:

'Hello, world'

Within a string, \ is an escape character, e.g. to include a quote:

In [14]:

"Hello, \"world\""

Out[14]:

'Hello, "world"'

String Operations¶

+ concatenates strings:

In [15]:

'hello' + 'world'

Out[15]:

'helloworld'

The split method separates a string into a list, by default on whitespace:

In [16]:

'hello world'.split()

Out[16]:

['hello', 'world']

Types¶

Python is strict about types - it won't auto-convert:

In [17]:

'maroon' + 5

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-17-bdfad87c9cc7> in <module>
----> 1 'maroon' + 5

TypeError: can only concatenate str (not "int") to str

We can convert to a string with the str function:

In [18]:

'maroon' + str(5)

Out[18]:

'maroon5'

Operations¶

We've now seen 3 different kinds of operations.

An operator:

In [19]:

6 + 7

Out[19]:

A function, in this case from a module:

In [20]:

np.log(10)

Out[20]:

2.302585092994046

A method, a function attached to an object:

In [21]:

'hello world'.split()

Out[21]:

['hello', 'world']

Lists¶

The split() method returned a list:

In [22]:

'hello world'.split()

Out[22]:

['hello', 'world']

We can write lists:

In [23]:

['martin', 'cross', 'gripps']

Out[23]:

['martin', 'cross', 'gripps']

More with Lists¶

Let's save a list in a variable:

In [24]:

rowdy3 = ['martin', 'cross', 'gripps']

We can add to the list:

In [25]:

rowdy3.append('vogel')
rowdy3

Out[25]:

['martin', 'cross', 'gripps', 'vogel']

List Members¶

Lists are indexed, starting with 0:

In [26]:

rowdy3[0]

Out[26]:

'martin'

They can be indexed backwards from the end:

In [27]:

rowdy3[-1]

Out[27]:

'vogel'

A slice takes multiple elements from a list:

In [28]:

rowdy3[1:3]

Out[28]:

['cross', 'gripps']

In [29]:

len(rowdy3)

Out[29]:

Loops¶

We can loop over a list:

In [30]:

for person in rowdy3:
    print(person)

martin
cross
gripps
vogel

Loops 2¶

Let's see 2 more tricks - including the position in our loop, and including variables in a string (an f-string):

In [31]:

for i, person in enumerate(rowdy3):
    print(f'Member {i}: {person}')

Member 0: martin
Member 1: cross
Member 2: gripps
Member 3: vogel

Loops 3¶

Python for loops iterate over an iterable, like a list.

To loop over a sequence of numbers, use a range:

In [32]:

for i in range(3):
    print(i)

0
1
2

Tuples¶

A Python tuple is like a list, except its size cannot be changed. Used for representing pairs, etc.:

In [33]:

coords = (3, 5)
coords[0]

Out[33]:

In [34]:

coords[1]

Out[34]:

A tuple can be unpacked:

In [36]:

x, y, z = coords
x

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-36-4ddec07835a7> in <module>
----> 1 x, y, z = coords
      2 x

ValueError: not enough values to unpack (expected 3, got 2)

Dictionaries¶

A dictionary maps keys — often strings — to values:

In [37]:

diets = {
    'rabbit': 'plants',
    'dog': 'meat',
    'vulture': 'carrion'
}

We can look up a value by its key:

In [38]:

diets['rabbit']

Out[38]:

'plants'

Objects¶

Everything in Python is an object, which has a type.

We've seen these types:

int
str (string)
list
tuple
dict (dictionary)

There is a lot more to do with these - see the readings!

References¶

Variables store references to objects. This matters for mutable objects:

In [39]:

rowdy3

Out[39]:

['martin', 'cross', 'gripps', 'vogel']

In [40]:

rowdy5 = rowdy3
rowdy5.append('amanda')
rowdy5

Out[40]:

['martin', 'cross', 'gripps', 'vogel', 'amanda']

But rowdy5 and rowdy3 are references to the same list object:

In [41]:

rowdy3

Out[41]:

['martin', 'cross', 'gripps', 'vogel', 'amanda']

Ways to Learn¶

These videos, but will not be complete
Textbook (Chapters 2–3 on Python)
Python Tutorial
Learn Python the Hard Way

Wrapping Up¶

Python supports many data types
Everything is an object
Variables store references to objects

Data Types and Operations