Week 3 Core Python Language¶

Mostly copied from the official python tutorial

1. Invoking Python¶

There are three main ways to use python.

1. By running a python file, e.g. python myscript.py
2. Through an interactive console (python interpreter or ipython shell)
3. In an interactive Jupyter Lab

We will be using the Jupyter Lab.

2. Basic Variables: Numbers and String¶

In [1]:

# comments are anything that comes after the "#" symbol
a = 1       # assign 1 to variable a
b = "hello" # assign "hello" to variable b

# comments are anything that comes after the "#" symbol a = 1 # assign 1 to variable a b = "hello" # assign "hello" to variable b

HintsThe following identifiers are used as reserved words, or keywords of the language, and cannot be used as ordinary identifiers. They must be spelled exactly as written here:

False      class      finally    is         return
None       continue   for        lambda     try
True       def        from       nonlocal   while
and        del        global     not        with
as         elif       if         or         yield
assert     else       import     pass
break      except     in         raise

Additionally, the following a built in functions which are always available in your namespace once you open a python interpreter

abs() dict() help() min() setattr() all() dir() hex() next() slice() any()
divmod() id() object() sorted() ascii() enumerate() input() oct() staticmethod()
bin() eval() int() open() str() bool() exec() isinstance() ord() sum() bytearray()
filter() issubclass() pow() super() bytes() float() iter() print() tuple()
callable() format() len() property() type() chr() frozenset() list() range()
vars() classmethod() getattr() locals() repr() zip() compile() globals() map()
reversed() __import__() complex() hasattr() max() round() delattr() hash()
memoryview() set()

In [2]:

# how to we see our variables?
print(a)
print(b)
print(a,b)

# how to we see our variables? print(a) print(b) print(a,b)

1
hello
1 hello

All variables are objects. Every object has a type (class). To find out what type your variables are

In [3]:

# as a shortcut, iPython notebooks will automatically print whatever is on the last line
type(b)

# as a shortcut, iPython notebooks will automatically print whatever is on the last line type(b)

Out[3]:

str

In [4]:

type(a) is int

type(a) is int

Out[4]:

True

Hints Different objects attributes and methods, which can be accessed via the syntax variable.method

IPython will autocomplete if you press <tab> to show you the methods available.

In [5]:

# this returns the method itself
b.capitalize

# this returns the method itself b.capitalize

Out[5]:

<function str.capitalize()>

In [6]:

# this calls the method
b.capitalize()
# there are lots of other methods

# this calls the method b.capitalize() # there are lots of other methods

Out[6]:

'Hello'

In [7]:

# binary operations act differently on different types of objects
c = 'World'
print(b + c)
print(a + 2)
print(a + b)

# binary operations act differently on different types of objects c = 'World' print(b + c) print(a + 2) print(a + b)

helloWorld
3

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[7], line 5
      3 print(b + c)
      4 print(a + 2)
----> 5 print(a + b)

TypeError: unsupported operand type(s) for +: 'int' and 'str'

Exercise 2:¶

Define a float variable d = 8. Print the type of the variable.

In [ ]:

3. Math¶

Basic arithmetic and boolean logic is part of the core python library.

In [8]:

# addition / subtraction
1+1-5

# addition / subtraction 1+1-5

Out[8]:

-3

In [9]:

# multiplication
5 * 10

# multiplication 5 * 10

Out[9]:

50

In [10]:

# division
1/2

# division 1/2

Out[10]:

0.5

In [11]:

# that was automatically converted to a float
type(1/2)

# that was automatically converted to a float type(1/2)

Out[11]:

float

In [12]:

# exponentiation
2**4

# exponentiation 2**4

Out[12]:

16

In [13]:

# rounding
round(9/10)

# rounding round(9/10)

Out[13]:

1

In [14]:

# built in complex number support
(1+2j) / (3-4j)

# built in complex number support (1+2j) / (3-4j)

Out[14]:

(-0.2+0.4j)

Exercise 3:¶

Let a = 2, b = 3.3, c = 2, calculate the following equation.

$y = 6a^3 - \frac{8b^2}{4c} + 11$

In [ ]:

4. Logic:¶

and: Returns True if both statements are true

or: Returns True if one of the statements is true

not: Reverse the result, returns False if the result is true

In [15]:

True and True

True and True

Out[15]:

True

In [16]:

True and False

True and False

Out[16]:

False

In [17]:

# logic

(not True) or (not False)

# logic (not True) or (not False)

Out[17]:

True

In [18]:

True or True

True or True

Out[18]:

True

In [19]:

x = 4
x < 5 and  x < 10

x = 4 x < 5 and x < 10

Out[19]:

True

In [20]:

x = 4
x < 5 or x < 4

x = 4 x < 5 or x < 4

Out[20]:

True

In [21]:

x = 4
x < 5 and x < 4

x = 4 x < 5 and x < 4

Out[21]:

False

In [22]:

not(x < 5 and x < 10)

not(x < 5 and x < 10)

Out[22]:

False

Exercise 4:¶

For each of the following expressions, guess whether they evaluate to True or False. Then type them to check your answers.

1. 1<=1
2. 1!=1
3. 123 == '123'
4. 1!=1 and 1<=1
5. 1!=1 or 1<=1
6. 'good' != 'bad'

In [ ]:

5. Conditionals¶

Conditional statements are an essential part of programming in Python. They allow you to make decisions based on the values of variables or the result of comparisons.

Hint: In Python, indentation is MANDATORY. Blocks are closed by indentation level.

In [50]:

x = 100
if x > 0:
    print('Positive Number')
elif x < 0:
    print('Negative Number')
else:
    print ('Zero!')

x = 100 if x > 0: print('Positive Number') elif x < 0: print('Negative Number') else: print ('Zero!')

Positive Number

In [51]:

# Blocks are closed by indentation level
if x > 0:
    print('Positive Number')
    if x >= 100:
        print('Huge number!')

# Blocks are closed by indentation level if x > 0: print('Positive Number') if x >= 100: print('Huge number!')

Positive Number
Huge number!

Exercise 5:¶

Given a number x, print if it is an odd or even number.

In [ ]:

6 Loop¶

A loop is an instruction that repeats multiple times as long as some condition is met. Loops are useful for iterating over sequences (like lists, strings, or ranges) or performing a task multiple times.

while loop¶

A while loop repeatedly executes a block of code as long as a specified condition is True.

In [52]:

# make a loop 
count = 0
while count < 10:
    # bad way
    # count = count + 1
    # better way
    count += 1
print(count)

# make a loop count = 0 while count < 10: # bad way # count = count + 1 # better way count += 1 print(count)

10

Hint: Be careful with the conditions. Avoid defining an infinite loop.

for loop¶

A for loop is used to iterate over a sequence (such as a list, tuple, string, or range) and execute a block of code for each item in that sequence.

In [53]:

# use range
for i in range(5):
    print(i)

# use range for i in range(5): print(i)

0
1
2
3
4

In [54]:

# Reverse the order
for i in range(5,0,-1):
    print(i)

# Reverse the order for i in range(5,0,-1): print(i)

5
4
3
2
1

Important point: in python, we always count from 0!

In [55]:

# what is range?
type(range)

# what is range? type(range)

Out[55]:

type

In [56]:

range?

range?

Init signature: range(self, /, *args, **kwargs)
Docstring:     
range(stop) -> range object
range(start, stop[, step]) -> range object

Return an object that produces a sequence of integers from start (inclusive)
to stop (exclusive) by step.  range(i, j) produces i, i+1, i+2, ..., j-1.
start defaults to 0, and stop is omitted!  range(4) produces 0, 1, 2, 3.
These are exactly the valid indices for a list of 4 elements.
When step is given, it specifies the increment (or decrement).
Type:           type
Subclasses:     

In [57]:

# iterate over a list we make up
for pet in ['dog', 'cat', 'fish']:
    print(pet, len(pet))

# iterate over a list we make up for pet in ['dog', 'cat', 'fish']: print(pet, len(pet))

dog 3
cat 3
fish 4

What is the thing in brackets? A list! Lists are one of the core python data structures.

Exercise:¶

Create a loop from 100 to 1, if odd number, print 'odd' and the number, or print 'even' and the number.

In [ ]:

7. Lists¶

In Python, a list is a built-in data type that allows you to store an ordered collection of items. These items can be of any data type, including integers, strings, floating-point numbers, or even other lists. Lists are mutable, meaning that you can change their content after they've been created (e.g., by adding, removing, or modifying elements).

In [58]:

l = ['dog', 'cat', 'fish']
type(l)

l = ['dog', 'cat', 'fish'] type(l)

Out[58]:

list

In [37]:

# list have lots of methods
l.sort()
l

# list have lots of methods l.sort() l

Out[37]:

['cat', 'dog', 'fish']

In [38]:

# we can convert a range to a list
r = list(range(5))
r

# we can convert a range to a list r = list(range(5)) r

Out[38]:

[0, 1, 2, 3, 4]

In [39]:

while r:
    p = r.pop()
    print('p:', p)
    print('r:', r)

while r: p = r.pop() print('p:', p) print('r:', r)

p: 4
r: [0, 1, 2, 3]
p: 3
r: [0, 1, 2]
p: 2
r: [0, 1]
p: 1
r: [0]
p: 0
r: []

There are many different ways to interact with lists. Exploring them is part of the fun of python.

list.append(x) Add an item to the end of the list. Equivalent to a[len(a):] = [x].

list.extend(L) Extend the list by appending all the items in the given list. Equivalent to a[len(a):] = L.

list.insert(i, x) Insert an item at a given position. The first argument is the index of the element before which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x).

list.remove(x) Remove the first item from the list whose value is x. It is an error if there is no such item.

list.pop([i]) Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list. (The square brackets around the i in the method signature denote that the parameter is optional, not that you should type square brackets at that position. You will see this notation frequently in the Python Library Reference.)

list.clear() Remove all items from the list. Equivalent to del a[:].

list.index(x) Return the index in the list of the first item whose value is x. It is an error if there is no such item.

list.count(x) Return the number of times x appears in the list.

list.sort() Sort the items of the list in place.

list.reverse() Reverse the elements of the list in place.

list.copy() Return a shallow copy of the list. Equivalent to a[:].

Don't assume you know how list operations work!

In [40]:

# "add" two lists
x = list(range(5))
y = list(range(10,15))
z = x + y
z

# "add" two lists x = list(range(5)) y = list(range(10,15)) z = x + y z

Out[40]:

[0, 1, 2, 3, 4, 10, 11, 12, 13, 14]

In [41]:

# access items from a list
print('first', z[0])
print('last', z[-1])
print('first 3', z[:3])
print('last 3', z[-3:])
print('middle, skipping every other item', z[5:10:2])

# access items from a list print('first', z[0]) print('last', z[-1]) print('first 3', z[:3]) print('last 3', z[-3:]) print('middle, skipping every other item', z[5:10:2])

first 0
last 14
first 3 [0, 1, 2]
last 3 [12, 13, 14]
middle, skipping every other item [10, 12, 14]

MEMORIZE THIS SYNTAX! It is central to so much of python and often proves confusing for users coming from other languages.

In terms of set notation, python indexing is left inclusive, right exclusive. If you remember this, you will never go wrong.

In [42]:

# that means we get an error from the following
N = len(z)
z[N]

# that means we get an error from the following N = len(z) z[N]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[42], line 3
      1 # that means we get an error from the following
      2 N = len(z)
----> 3 z[N]

IndexError: list index out of range

In [43]:

# this index notation also applies to strings
name = 'Xiaomeng Jin'
print(name[:4])

# this index notation also applies to strings name = 'Xiaomeng Jin' print(name[:4])

Xiao

In [44]:

print(name[:-4])

print(name[:-4])

Xiaomeng

In [45]:

print(name[-3:])

print(name[-3:])

Jin

In [46]:

# you can also test for the presence of items in a list
5 in z

# you can also test for the presence of items in a list 5 in z

Out[46]:

False

Lists are not meant for math! They don't have a datatype.

In [47]:

z[4] = 'fish'
z

z[4] = 'fish' z

Out[47]:

[0, 1, 2, 3, 'fish', 10, 11, 12, 13, 14]

Python is full of tricks for iterating and working with lists

In [48]:

# a cool python trick: list comprehension
squares = [n**2 for n in range(5)]
squares

# a cool python trick: list comprehension squares = [n**2 for n in range(5)] squares

Out[48]:

[0, 1, 4, 9, 16]

In [49]:

# iterate over two lists together uzing zip
for item1, item2 in zip(x,y):
    print('first:', item1, 'second:', item2)

# iterate over two lists together uzing zip for item1, item2 in zip(x,y): print('first:', item1, 'second:', item2)

first: 0 second: 10
first: 1 second: 11
first: 2 second: 12
first: 3 second: 13
first: 4 second: 14

Exercise 7:¶

Define a list from a range of values, check if an element exists in the list.

In [ ]:

8. Assignment: Python Lists and Loops¶

In this problem, we will explore the basic data structures and flow controls of python by manually parsing a CSV file.

Note that this is a futile exercise. In the "real world" you should never manually parse a CSV file. There are utilities out there that will do it for you much more quickly and efficiently. However, it is a useful exercise for learning python.

Before starting the python part, use the JupyterLab file browser to browse to this file. Click to open it. What do you see?

Now we will begin the process of reading the file with python

8.1 Open the file using the open function¶

Specifically, run the command

file = open('esa_roster_fall_2024.csv')

In [ ]:

8.2 Use the help function to get the documentation for your new variable file¶

This will produce a long list of methods you can use with file.

In [ ]:

8.3 Read the lines of the file into a variable called lines¶

Hint: use the documentation above to find the method that sounds most likely to do what you want.

In [ ]:

It should be a familiar type we learned about in class.

8.4 Display lines at the end of a cell in order to see its contents¶

In [ ]:

8.5 Display the number of students in class¶

In [ ]:

8.6 Use slicing to display the first three items of the list. And the last 3¶

In [ ]:

8.7 Now iterate through lines and print the item if it contains your NetID¶

In [ ]: