Ch03 Getting Started with Python¶

Data Types¶

Variables¶

a = 3
b = 4
a + b

a = 3
print(a)
a = "three"
print(a)

3
three

Numeric Types¶

type(4)

int

type(4.4)

float

type(4.)

float

float(4)

4.0

int(4.9)

1.125 - 1.1

0.02499999999999991

3 + 4  # Sum

3 - 4  # Subtraction

-1

3 / 4  # Division

0.75

3 * 4  # Multiplication

3**4  # The power operator (Excel uses 3^4)

3 * (3 + 4)  # Use of parentheses

Comments¶

# This is a sample we've seen before.
# Every comment line has to start with a #
3 + 4

3 + 4  # This is an inline comment

Booleans¶

3 == 4  # Equality (Excel uses 3 = 4)

False

3 != 4  # Inequality (Excel uses 3 <> 4)

True

3 < 4  # Smaller than. Use > for bigger than.

True

3 <= 4  # Smaller or equal. Use >= for bigger or equal.

True

# You can chain logical expressions
# In VBA, this would be: 10 < 12 And 12 < 17
# In Excel formulas, this would be: =AND(10 < 12, 12 < 17)
10 < 12 < 17

True

not True  # "not" operator

False

False and True  # "and" operator

False

False or True  # "or" operator

True

bool(2)

True

bool(0)

False

bool("some text")  # We'll get to strings in a moment

True

bool("")

False

bool(None)

False

Strings¶

"A double quote string. " + 'A single quote string.'

'A double quote string. A single quote string.'

print("Don't wait! " + 'Learn how to "speak" Python.')

Don't wait! Learn how to "speak" Python.

print("It's easy to \"escape\" characters with a leading \\.")

It's easy to "escape" characters with a leading \.

# Note how Python allows you to conveniently assign multiple
# values to multiple variables in a single line
first_adjective, second_adjective = "free", "open source"
f"Python is {first_adjective} and {second_adjective}."

'Python is free and open source.'

"PYTHON".lower()

'python'

"python".upper()

'PYTHON'

Indexing and Slicing¶

language = "PYTHON"

language[0]

'P'

language[1]

'Y'

language[-1]

'N'

language[-2]

'O'

language[:3]  # Same as language[0:3]

'PYT'

language[1:3]

'YT'

language[-3:]  # Same as language[-3:6]

'HON'

language[-3:-1]

'HO'

language[::2]  # Every second element

'PTO'

language[-1:-4:-1]  # Negative step goes from right to left

'NOH'

language[-3:][1]

'O'

Data Structures¶

Lists¶

file_names = ["one.xlsx", "two.xlsx", "three.xlsx"]
numbers = [1, 2, 3]

file_names + numbers

['one.xlsx', 'two.xlsx', 'three.xlsx', 1, 2, 3]

nested_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

cells = [[1, 2, 3],
         [4, 5, 6],
         [7, 8, 9]]

cells[1]  # Second row

[4, 5, 6]

cells[1][1:]  # Second row, second and third column

[5, 6]

Line Continuation¶

a = (1 + 2
     + 3)

a = 1 + 2 \
    + 3

users = ["Linda", "Brian"]

users.append("Jennifer")  # Most commonly you add to the end
users

['Linda', 'Brian', 'Jennifer']

users.insert(0, "Kim")  # Insert "Kim" at index 0 
users

['Kim', 'Linda', 'Brian', 'Jennifer']

users.pop()  # Removes and returns the last element by default

'Jennifer'

users

['Kim', 'Linda', 'Brian']

del users[0]  # del removes an element at the given index

len(users)  # Length

"Linda" in users  # Check if users contains "Linda"

True

print(sorted(users))  # Returns a new sorted list
print(users)  # The original list is unchanged

['Brian', 'Linda']
['Linda', 'Brian']

users.sort()  # Sorts the original list
users

['Brian', 'Linda']

len("Python")

"free" in "Python is free and open source."

True

Dictionaries¶

exchange_rates = {"EURUSD": 1.1152,
                  "GBPUSD": 1.2454,
                  "AUDUSD": 0.6161}

exchange_rates["EURUSD"]  # Access the EURUSD exchange rate

1.1152

exchange_rates["EURUSD"] = 1.2  # Change an existing value
exchange_rates

{'EURUSD': 1.2, 'GBPUSD': 1.2454, 'AUDUSD': 0.6161}

exchange_rates["CADUSD"] = 0.714  # Add a new key/value pair
exchange_rates

{'EURUSD': 1.2, 'GBPUSD': 1.2454, 'AUDUSD': 0.6161, 'CADUSD': 0.714}

{**exchange_rates, **{"SGDUSD": 0.7004, "GBPUSD": 1.2222}}

{'EURUSD': 1.2,
 'GBPUSD': 1.2222,
 'AUDUSD': 0.6161,
 'CADUSD': 0.714,
 'SGDUSD': 0.7004}

currencies = {1: "EUR", 2: "USD", 3: "AUD"}

currencies[1]

'EUR'

# currencies[100] would raise an exception. Instead of 100,
# you could use any other non-existing key, too.
currencies.get(100, "N/A")

'N/A'

Tuple¶

currencies = ("EUR", "GBP", "AUD")

currencies[0]  # Accessing the first element

'EUR'

# Concatenating tuples will return a new tuple.
currencies + ("SGD",)

('EUR', 'GBP', 'AUD', 'SGD')

Set¶

set(["USD", "USD", "SGD", "EUR", "USD", "EUR"])

{'EUR', 'SGD', 'USD'}

portfolio1 = {"USD", "EUR", "SGD", "CHF"}
portfolio2 = {"EUR", "SGD", "CAD"}

# Same as portfolio2.union(portfolio1)
portfolio1.union(portfolio2)

{'CAD', 'CHF', 'EUR', 'SGD', 'USD'}

# Same as portfolio2.intersection(portfolio1)
portfolio1.intersection(portfolio2)

{'EUR', 'SGD'}

Summary¶

currencies = "USD", "EUR", "CHF"
currencies

('USD', 'EUR', 'CHF')

list(currencies)

['USD', 'EUR', 'CHF']

Control Flow¶

If Statement¶

i = 20
if i < 5:
    print("i is smaller than 5")
elif i <= 10:
    print("i is between 5 and 10")
else:
    print("i is bigger than 10")

i is bigger than 10

is_important = True
if is_important:
    print("This is important.")
else:
    print("This is not important.")

This is important.

values = []
if values:
    print(f"The following values were provided: {values}")
else:
    print("There were no values provided.")

There were no values provided.

is_important = False
print("important") if is_important else print("not important")

not important

For and While Loops¶

currencies = ["USD", "HKD", "AUD"]

for currency in currencies:
    print(currency)

USD
HKD
AUD

range(5)

range(0, 5)

list(range(5))  # stop argument

[0, 1, 2, 3, 4]

list(range(2, 5, 2))  # start, stop, step arguments

[2, 4]

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

0
1
2

for i, currency in enumerate(currencies):
    print(i, currency)

USD
HKD
AUD

exchange_rates = {"EURUSD": 1.1152,
                  "GBPUSD": 1.2454,
                  "AUDUSD": 0.6161}
for currency_pair in exchange_rates:
    print(currency_pair)

EURUSD
GBPUSD
AUDUSD

for currency_pair, exchange_rate in exchange_rates.items():
    print(currency_pair, exchange_rate)

EURUSD 1.1152
GBPUSD 1.2454
AUDUSD 0.6161

for i in range(15):
    if i == 2:
        break
    else:
        print(i)

0
1

for i in range(4):
    if i == 2:
        continue
    else:
        print(i)

0
1
3

for i in range(1, 4):
    print(i)
print(i)

n = 0
while n <= 2:
    print(n)
    n += 1

0
1
2

List, Set and Dictionary Comprehensions¶

currency_pairs = ["USDJPY", "USDGBP", "USDCHF",
                  "USDCAD", "AUDUSD", "NZDUSD"]

usd_quote = []
for pair in currency_pairs:
    if pair[3:] == "USD":
        usd_quote.append(pair[:3])
usd_quote

['AUD', 'NZD']

[pair[:3] for pair in currency_pairs if pair[3:] == "USD"]

['AUD', 'NZD']

[pair[3:] + pair[:3] for pair in currency_pairs]

['JPYUSD', 'GBPUSD', 'CHFUSD', 'CADUSD', 'USDAUD', 'USDNZD']

exchange_rates = {"EURUSD": 1.1152,
                  "GBPUSD": 1.2454,
                  "AUDUSD": 0.6161}
{k: v * 100 for (k, v) in exchange_rates.items()}

{'EURUSD': 111.52, 'GBPUSD': 124.54, 'AUDUSD': 61.61}

{s + "USD" for s in ["EUR", "GBP", "EUR", "HKD", "HKD"]}

{'EURUSD', 'GBPUSD', 'HKDUSD'}

Code organization¶

Functions¶

def convert_to_celsius(degrees, source="fahrenheit"):
    if source.lower() == "fahrenheit":
        return (degrees-32) * (5/9)
    elif source.lower() == "kelvin":
        return degrees - 273.15
    else:
        return f"Don't know how to convert from {source}"

convert_to_celsius(100, "fahrenheit")  # Positional arguments

37.77777777777778

convert_to_celsius(50)  # Will use the default source (fahrenheit)

10.0

convert_to_celsius(source="kelvin", degrees=0)  # Keyword arguments

-273.15

Modules and the Import Statement¶

import temperature

This is the temperature module.

temperature.TEMPERATURE_SCALES

('fahrenheit', 'kelvin', 'celsius')

temperature.convert_to_celsius(120, "fahrenheit")

48.88888888888889

import temperature as tp

tp.TEMPERATURE_SCALES

('fahrenheit', 'kelvin', 'celsius')

from temperature import TEMPERATURE_SCALES, convert_to_celsius

TEMPERATURE_SCALES

('fahrenheit', 'kelvin', 'celsius')

Date and Time¶

# Import the datetime module as "dt"
import datetime as dt

# Instantiate a datetime object called "timestamp"
timestamp = dt.datetime(2020, 1, 31, 14, 30)
timestamp

datetime.datetime(2020, 1, 31, 14, 30)

# Datetime objects offer various attributes, e.g. to get the day
timestamp.day

# The difference of two datetime objects returns a timedelta object
timestamp - dt.datetime(2020, 1, 14, 12, 0)

datetime.timedelta(days=17, seconds=9000)

# Accordingly, you can also work with timedelta objects
timestamp + dt.timedelta(days=1, hours=4, minutes=11)

datetime.datetime(2020, 2, 1, 18, 41)

# Format a datetime object in a specific way
# You could also use an f-string: f"{timestamp:%d/%m/%Y %H:%M}"
timestamp.strftime("%d/%m/%Y %H:%M")

'31/01/2020 14:30'

# Parse a string into a datetime object
dt.datetime.strptime("12.1.2020", "%d.%m.%Y")

datetime.datetime(2020, 1, 12, 0, 0)

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Ch03 Getting Started with Python¶

Data Types¶

Variables¶

Numeric Types¶

Comments¶

Booleans¶

Strings¶

Indexing and Slicing¶

Data Structures¶

Lists¶

Line Continuation¶

Dictionaries¶

Tuple¶

Set¶

Summary¶

Control Flow¶

If Statement¶

For and While Loops¶

List, Set and Dictionary Comprehensions¶

Code organization¶

Functions¶

Modules and the Import Statement¶

Date and Time¶