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How do you open and read a text file using the 'with' statement, and why is 'with' preferred over manual open()/close()? What are the common file modes ('r', 'w', 'a', 'rb') passed to open(), and what does each one do? How do you write and use a custom context manager by implementing __enter__ and __exit__? How does @contextlib.contextmanager let you write a context manager using a generator function instead of a full class? How do you use pathlib.Path for modern, object-oriented file path manipulation instead of os.path's string-based functions? How do you use multiple context managers in a single 'with' statement, and how does this compare to nesting separate 'with' blocks? How do you check whether a file or directory exists before operating on it, using pathlib? How would you implement a context manager that temporarily changes the current working directory and restores it afterward? How do you implement contextlib.ExitStack to dynamically manage a VARIABLE number of context managers determined at runtime? How would you write a reusable context manager decorator using contextlib.contextmanager that ALSO works when applied directly to a function, like @contextmanager-decorated functions support?
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What is the difference between reading a file with .read(), .readline(), and .readlines()?

Beginner

Answer

read() returns the ENTIRE file's contents as a single string; readline() returns just the NEXT single line (including its trailing newline); readlines() returns a LIST of every line in the file — read() and readlines() both load the whole file into memory at once.
with open("data.txt") as f:
    all_content = f.read()        # one big string

with open("data.txt") as f:
    first_line = f.readline()      # just one line

with open("data.txt") as f:
    lines = f.readlines()          # a list of every line
Real-world example Choosing readline() to process a huge file one line at a time versus read() for a small file you need entirely at once.

Common follow-ups: How does iterating directly over the file object (for line in f) compare to calling readlines()?

Memory Management & Garbage Collection
All 15 File I/O & Context Managers questions