CS50x Section 6 - Python

Python is an interpreted, object-oriented, high-level programming language. Its syntax improves readability and its extensive standard library and other third-party libraries simplify the programming process.

Strings

// Strings in C

char *text = get_string("...");

# Strings in Python

text = input("...")

Notice the semicolon :, the type declaration char * are not needed in Python.

// Comparing strings in C

if (strcmp(text, "hello") == 0)
{
    ...
}

# Comparing strings in Python

if text == "hello":
    ...

Notice the curly braces {} and parenthesis () are not needed, Python uses indentation.
We also do not need to call our own function strcmp.

// Indexing into string's characters in C

text[i]

# Indexing into string's characters in Python

text[i]

Indexing into a string is the same in C and in Python.

Dot Notation - `strip()`

Imagine prompting the user for a string:

text = input("...")

" In the great green room  "

Notice that the user's string is not clean, it has extra spaces, that could cause problems when working with the data.

text = input("...")
text.strip()

"In the great green room"

Applying the strip() function to the text, will clean the data by taking out the beginning and trailing white spaces.
Notice the dot notation syntax text.strip().

`lower()`

text = input("...")

" IN thE great green rOom  "

Notice that the user's string is not clean, it is not capitalized properly.

text = input("...")
text.lower()

"in the great green room"

Applying the lower() function to the text, will clean the data by making all characters lowercase.
Notice the dot notation syntax text.lower().

`capitalize()`

text = input("...")

" IN thE great green rOom  "

Notice that the user's string is not clean, it is not capitalized properly.

text = input("...")
text.capitalize()

"In the great green room"

Applying the capitalize() function to the text, will clean the data by making the first character only.
Notice the dot notation syntax text.capitalize().

Caution

When using text.capitalize(), to apply the Capitalization to the string, it is necessary to reassign the result back to the variable like so text = text.capitalize().

`str`

Unlike in C, Python strings are objects that not only represent the value they hold (sequence of characters), but also have associated behaviors through functions or methods. The Python documentation lists all the built-in string methods.

Loops

text = "In the great green room"

for c in text:
    print(c)

This loop iterates through every character of the string In the great green room and prints it one character per line.

text = "In the great green room"

words = text.split()

['In', 'the', 'great', 'green', 'room']

The method split() will split the string into elements separated by the whitespace.

text = "In the great green room"

words = text.split()
for word in words:
    print(word)

In 
the
great
green
room

The method split() is used to split the string into elements separated by whitespace.
The loop will iterate through the resulting list of words printing each of them word in a new line.

Notice that we did not have to define the new elements word or the list words. Python's for/in syntax helps you iterate through components of an iterable (string, list, etc.) while referring to them by a convenient name.

Let's combine different loops and strings syntax and see what the output would be:

text = "In the great green room"

for word in text.split():
    print(word)

In
the
great
green
room

Notice that in Python the default behavior of a print statement is to automatically append a new line.
We can override this with the end=" " named parameter print(word, end=" ").

text = "In the great green room"

for word in text.split():
    for c in word:
        print(c)

I
n
t
h
r
g
r
e
...

Notice that there are no more spaces, text.split() removed the spaces
First loop iterates through the words, while the indented loop iterates through each character c and prints it.

text = "In the great green room"

for word in text.split():
    if "g" in word:
        print(word)

great
green

The loop iterates through every element word the list, and prints it if in contains character g (implements linear search).

text = "In the great green room"

for word in text.split()[2:]:
    print(word)

great
green
room

This code uses bracket notation [] to index into a specific element of the list. Remember how we used word[0] to access the 1st element, word[1] to access the 2nd element, etc.
This new syntax [2:] means to index into the second element of the list 2 and the colon : means to include the following elements of the list after that point.
The loop basically starts iterating through the list at element[2].

Note

This special syntax allows us to decide which elements of the list we want to include. list[inclusive : exclusive]. Let's imagine this scenario list[1:4].
This means to start including elements at index 1 and exclude element 4 and beyond, resulting in a list of elements 1, 2 and 3.

Dictionaries

Dictionaries are unordered, mutable data structures that allow you to store key-value pairs, where each key is unique and used to access its corresponding value.

Let's see an example of a dictionary called authors where the key is the title of a book, and the value the author:

Key	Value
Goodnight Moon	Margaret Wise Brown
Corduroy	Don Freeman
Curious George	H.A. Ray

We could also create one dictionary per book, holding information in several key-value combinations:

Key	Value
Title	Goodnight Moon
Author	Margaret Wise Brown

# In Python 

book = dict()

book["title"] = "Corduroy"
book["author"] = "Don Freeman"

Declares a dictionary called book.
book["title"] Declares the key.
= "Corduroy" Assigns the value to the key.

book = dict()

book["title"] = "Corduroy"
book["author"] = "Don Freeman"

print(book["title"])

Corduroy

book["title"] Placing the key inside the print statement will print the associated value of the book.

book = dict()

book["title"] = "Corduroy"
book["author"] = "Don Freeman"

print(book["Corduroy"])

KeyError: 'Corduroy'

Notice in the print statement, we put the value Corduroy inside the brackets [] instead of the key title. Attempting to print a value this way will result in an error KeyError.

Let's explore different syntax for creating dictionaries:

# Alternative syntax

book = {
    "title": "Goodnight Moon",
    "author": "Margaret Wise Brown"
}

This syntax allows us to create the dictionary and enter all the data upfront.

List of Dictionaries

We could also create a list of dictionaries:

books = [{"title": "Goodnight Moon",...},
{"title": "Corduroy",...},
{"title": "Curious George",...}]

Inside the brackets [] is a list of dictionaries.
Individual dictionaries are inside the curly braces {} and separated by comas ,. Different dictionaries can have the same key.

Title (key)	Author (Value)
Goodnight Moon	Margaret Wise Brown
Corduroy	Don Freeman
Curious George	H.A. Ray

# Create an empty list of books
books = []

# Add three books to shelf
for i in range(3):
    book = dict()
    book["author"] = input("Enter an author: ")
    book["title"] = input("Enter a title: ")
    books.append(book)

# Print list of books
for book in books:
    print(book)

{'author': 'Margaret Wise Brown', 'title': 'Goodnight Moon'}
{'author': 'Don Freeman', 'title': 'Corduroy'}
{'author': 'H.A. Ray', 'title': 'Curious George'}

The loop creates 3 dictionaries called book by prompting the user for the key author and the value title and appends the book to the list called books.
Notice the syntax books.append(book). The method append() will add elements to the end of the list. list.append(element).
the second loop will print each dictionary book in a different line with its keys and values between {}.

Alternative print statement:

# Print list of books
for book in books:
    print(f"{book['author']} wrote {book['title']}.")

Margaret Wise Brown wrote Goodnight Moon
Don Freeman wrote Corduroy
H.A. Ray wrote Curious George

Notice the f f-string syntax in the print statement, allowing us to embed the keys inside string literals {book['author']} to access the values.

Libraries and Modules

In the previous example we saw how to prompt the user to enter data manually, but this method can be strenuous when using a large data set. To make this processes more efficient we can also read data from a spreadsheet.

Spreadsheet:

Title (key)	Author (Value)
Goodnight Moon	Margaret Wise Brown
Corduroy	Don Freeman
Curious George	H.A. Ray
Winnie-the-pooh	A.A. Milne
Fantastic Mr. Fox	Roald Dahl
Charlotte's Web	E.B. White
...	...

Export the data from the spreadsheet into a CSV (Comma-Separated Values) file, a common file format used for it's simplicity and portability.

books.csv:

title,author
Goodnight Moon,Margaret Wise Brown
Corduroy,Don Freeman
Curious George,H.A. Ray
Winnie-the-pooh,A.A. Milne
Fantastic Mr. Fox,Roald Dahl
Charlotte's Web,E.B. White
...,...

Read/Write from the CSV file. Python has a built-in CSV module that provides methods for reading and writing.
```
import csv
```
- Placing this line at the top of our Python file will import the entire CSV module, including all of its methods (DictReader(), DictWriter(), reader(), writer(), etc.).
```
csv.DictReader(...)
csv.reader(...)
```
- To access a specific method in the CSV module we have to use the dot notation module prefix.method()
We could also choose to only import a specific method in the CSV module:
```
from csv import DictReader
```
- Placing this line at the top of our Python file will only import the DictReader() method from the CSV module.
```
from csv import DictReader

DictReader(...)
```
- Notice that we did not have to include the module prefix csv. when calling DictReader(). This is because we imported the method directly.

File I/O

Python provides powerful and efficient functions for file handling. These functions allow us to open, read from, write to and close files.

Opening a file:
```
with open(FILENAME) as file:
```
- with starts the context manager that will manage the file resource.
- open() opens the file FILENAME in read mode by default.
- as file Assigns the opened file object to the variable file (to be used to perform operation on the file).

Reading from a file:
```
# Reading from a text file

with open(FILENAME) as file:
    text = file.read()
```
- The code inside the with block will be executed with the file open and assigned to the file variable. (with will also automatically close the file as soon as we are no longer indented in the block).
- text = file.read() Reads the entire data in the file into the variable text.
- This works really well for plain text .txt files.
For CSV files, it is better suited to use Python's CSV module:
```
# Reading from a csv file

with open(FILENAME) as file:
    file_reader = csv.DictReader(file)
    for row in file_reader:
        ...
```
- file_reader = csv.DictReader(file) Creates a DictReader object named file_reader, that reads each row of the CSV file as a dictionary.
- The keys of the dictionary are derived from the first row (header) of the csv file.
- for row in file_reader This line starts a loop that will iterate over each row (dictionary) in the file_reader.

`reads.py`

import csv

with open("books.csv") as file:
    text = file.read(file)
    print(text)

title,author
Goodnight Moon,Margaret Wise Brown
Corduroy,Don Freeman
Curious George,H.A. Ray
Winnie-the-Pooh,A.A. Milne
Fantastic Mr. Fox,Roald Dahl
Charlotte's Web,E.B. White
...

Notice that reading from a CSV file with read() will read all the content of the file row by row.

Now, let's use CSV module specific methods:

import csv

with open("books.csv") as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(row)

{'title': 'Goodnight Moon', 'author': 'Margaret Wise Brown'}
{'title': 'Corduroy','author': 'Don Freeman'}
{'title': 'Curious George', 'author': 'H.A. Ray'}
{'title': 'Winnie-the-Pooh', 'author': 'A.A. Milne'}
{'title': 'Fantastic Mr. Fox', 'author': 'Roald Dahl'}
{'title': 'Charlotte's Web', 'author': 'E.B. White'}

Notice now that reading from the same CSV file using csv.DictReader will create dictionaries for each row using the first row title, author to make the keys.

Let's complete the program above:

import csv

books = []

# Add books to the shelf by reading from books.csv
with open("books.csv") as file:
    reader = csv.DictReader(file)
    for row in reader:
        books.append(row)

# Print books
for book in books:
    print(book)

Import the csv module.
Create an empty list named books.
Open books.csv content and store it in file.
Read from file using DictReader(), which will create a iterable object of dictionaries, one for each row of file. Store this iterable in object reader.
Loop through each row of reader (each dictionary) and use append() to add it to the list named books.
Print each element (book) of the books list.

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section6.md

section6.md

CS50x Section 6 - Python

Strings

Dot Notation - `strip()`

`lower()`

`capitalize()`

`str`

Loops

Dictionaries

List of Dictionaries

Libraries and Modules

File I/O

`reads.py`

Files

section6.md

Latest commit

History

section6.md

File metadata and controls

CS50x Section 6 - Python

Strings

Dot Notation - strip()

lower()

capitalize()

str

Loops

Dictionaries

List of Dictionaries

Libraries and Modules

File I/O

reads.py

Dot Notation - `strip()`

`lower()`

`capitalize()`

`str`

`reads.py`