Unit 6 - File I/O And Strings

String Definition

Strings are ordered sequences of characters

String Indexing

Accessing a single character at a specific position

String Slicing

Extracting a subsequence of characters

String Concatenation

Joining strings together using the + operator

String Repetition

Repeating a string multiple times using the * operator

String Length

Getting the number of characters using len()

String Immutability

Strings are immutable; operations create new strings rather than altering existing ones

ASCII

The original standard, using 7 bits to represent 128 characters - this includes English letters, numbers, punctuation, and control characters

Unicode

A much larger standard that encompasses characters from virtually all writing systems, as well as symbols and emojis

ord(character)

Returns the integer Unicode code of the character

chr(integer)

Returns the character corresponding to an integer Unicode code

Substring Membership (in)

Check if a substring exists within a larger string

Finding Substrings (find())

Returns the starting index of the first occurrence of a substring - if the substring is not found, it returns -1

Replacing Substrings (replace())

Returns a new string where all occurrences of a specified substring are replaced with another substring

.lower()

Converts all characters to lowercase

.upper()

Converts all characters to uppercase

.capitalise()

Capitalises the first character of the string.

.title()

Capitalises the first character of each word

.swapcase()

Swaps the case of all characters

.ljust(n)

Left-justifies the string in a field of width n, padding with spaces on the right

.rjust(n)

Right-justifies the string in a field of width n, padding with spaces on the left

.center(n)

Centres the string in a field of width n, padding with spaces on both sides

Splitting (split())

Breaks a string into a list of substrings based on a specified delimiter - if no delimiter is provided, it splits on any whitespace character (space, tab, newline) and discards empty strings

Joining (join())

Takes an iterable (like a list) of pixels and concatenates them into a single string, using the string it’s called on as a separator

Joining Syntax

separator.join(iterable)

Splitting Lines (splitlines())

A convenient way to split a string into a list of lines, respecting different newline characters

Efficiency Of Combining Strings

For joining a large number of strings, join() is significantly more performant than using the + operator repeatedly, as + creates many intermediate string objects

Persistence In Files

Data in memory is lost when a program ends or the computer turns off - files save data so it can be used later

Interchange In Files

Files allow different programs to communicate and share data

Organisation In Files

File systems (directories and folders) provide a hierarchical structure for organising information

Large Data In Files

Files enable working with datasets that are too large to fit into the computer’s main memory

Files

Files provide a mechanism for long-term data storage, inter-program communication, and structured organisation

Reading Files

Fetching data sequentially from the file

Writing To Files

Appending data sequentially to the file

Seek (In Files)

Moving the current position within the file

State (In Files)

Streams have a “position” that tracks how far you are into the file - reading or writing updates this position

Context Managers (with open(…) as f:)

It automatically ensures that the file is properly closed after you’re done with it, even if errors occur - this is crucial for releasing system resources

“r”

Read Mode (default) - opens a file for reading - if the file doesn’t exist, it raises a FileNotFoundError

“w”

Write Mode - opens a file for writing - crucially, this will erase the contents of the file if it already exists - if the file doesn’t exist, it creates a new one

“a”

Append Mode - opens a file for writing, but adds new data to the end of the file - if the file doesn’t exist, it creates a new one

“r+”

Read/Write Mode - opens a file for both reading and writing

read()

Reads the entire content of the file into a single string - be cautious with very large files, as this can consume a lot of memory

file.read()

Reads from the current position to the end of the file

file.read(n)

Reads at most n characters from the file

readLine()

Reads a single line from the file, including the newline character (\n) at the end (if present) - it returns an empty string when the end of the file is reached

readlines()

Reads all lines from the file and returns them as a list of strings - each string in the list includes the trailing newline character

Natural Iteration (using a for loop)

This is often the most Pythonic and memory-efficient way to process as file line by line, especially for large files - the file object itself acts as an iterator

write(s)

Writes a string s to the file at the current position

writelines(list_of_strings)

Writes each string from the provided list (or any iterable) to the file - similar to write(), it does not add newlines automatically

print(…, file=f)

Write formatted output to a file, mimicking the behaviour of the standard print() function - it automatically adds a newline character at the end unless specified otherwise

strip()

This string method removes all leading and trailing whitespace characters from a string

lstrip()

Removes only leading (left-side) whitespace

rstrip()

Removes only trailing (right-side) whitespace