Files

Storage

• Instructions are executed by the CPU
• Values and variables as discussed so far live in main memory

Persistence

• Main memory is cleared when unpowered
• Long-term storage must use secondary memory

Files

• Files are the abstraction used for long-term storage
• Files are typically organized in a hierarchy
• Files typically have human-readable names

Opening Files

• The open function may be used to open a file
• It requires a filename as a parameter
• It returns a file handle

Read Example

file_handle = open("myfile.txt")

contents = file_handle.read()

print(contents)

Closing Files

• When we are finished with a file, we should close it to free resources in our programs

Close Example

file_handle = open("myfile.txt")

contents = file_handle.read()

file_handle.close()

print(contents)

Reading Closed File

file_handle = open("myfile.txt")

file_handle.close()

contents = file_handle.read() # Raises Exception

Lines

• Plain text may be separated into lines for easier consumption
• Lines are separated by a special character called a newline
• We can create a newline character in Python using a \n escape sequence

Example

print("Line 1\nLine 2")

Reading Lines

• It may be helpful to read a file one line at a time
• Paragraphs may be represented this way in documents
• Data formats may use lines to separate records

Reading Lines

• The readline method will return the next line as a string value
• The readlines method will return an iterable of all lines
• The file handle can be iterated directly to operate on lines

Example

handle = open("example.py")

first_line = handle.readline()

print(first_line)

Example

handle = open("example.py")

for line in handle:
    print(line, end='')

Exercise

1. Create a plain text file with numbers on each line
2. Create a Python program that prints the sum of the numbers in the file

Solution

handle = open("myfile.txt")

total = 0
for line in handle:
    total += int(line)

print(total)

Key Ideas

• Main memory is volatile and files provide non-volatile storage
• We can use the open function to get a file handle
• We can use read to load a string from a file