File Processing

A file is a collection of data stored in a structured format. We generally want to either read (load) data from a file or write (save) data to a file.

A file system is a hierarchical structure that organizes files using (nested) folders called directories. In most computer systems, files are stored in a specific directory with a unique file name. For example, the current lectures directory includes all the lectures for the course, each of which is represented by a .md (Markdown) file.

A path specifies a particular file in a particular directory. For example, the absolute path ~/COURSE_MATERIALS/lectures/file-processing.md consists of:

• The ~ (tilde), which represents the current user’s home directory.

• The subdirectory COURSE_MATERIALS/lectures/ within the user’s home directory.

• The file named file-processing.md within that subdirectory.

This uniquely identifies this file! But we could have also specified this as a relative path ./file-processing.md, which consists of:

• The . (period), which represents the current directory.

• The file named file-processing.md within that directory.

We can even omit the ./ at the beginning leaving just the file name file-processing.md. This will also search for file-processing.md within the current directory.

Reading files¶

To read a file in Python, first call the open function and specify the file path.

f = open("data/file.fl")

To read the entire contents of the file into a single string:

f.read()

Alternatively, to read the file one line at a time, we can iterate over the file object using a for loop:

for line in f:
    print(line)

Practice: Nominees¶

Write code that reads the contents of movie nominees.txt and evaluates to a list containing each movie name.

Frankenstein
Sinners
F1
Hamnet
Bugonia
Sentimental Value
Train Dreams
The Secret Agent
Marty Supreme
One Battle After Another

Writing files¶

To write a file from Python back to the computer file system, first call the open function with the "w" (write) mode argument:

f = open("data/file.fl", "w")
f.write(string)

Alternatively, to append to content to the end of a file, call the open function with the "a" (append) mode argument:

f = open("data/file.fl", "a")
f.write(string)

In append mode, existing file contents are not overwritten; new content is just added to the end.

Closing files¶

We’re not done yet! When you are finished working with a file, it’s important to close it:

f.close()

Changes to a file may not be fully implemented or saved until close is called. Furthermore, keeping a file open may prevent other applications from using or modifying the file.

Alternatively, we can use the with statement to automatically close resources at the end of the block:

with open("data/file.fl") as f:
    print(f.read())

Example: Writing in read mode¶

What happens if we try to write to a file opened in read mode?

with open("data/file.fl") as f:
    print(f.read())
    f.write("New content :)")

Practice: Rainfall¶

How rainy is Seattle? Let’s analyze rainfall statistics collected by a sensor that can sometimes make mistakes and report negative rainfall.

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8
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Write a program to print a single line summarizing statistics about a rainfall data file:

• the sum total rainfall (only the positive integers),

• the number of positive integers, and

• the average rainfall.

If there are no positive integers in the file, print “No rain” instead of these three values. Use the int function to convert a string to an integer.