PCEP: File Handling and Module Imports

What this domain covers

The smallest PCEP domain covers opening, reading and writing text files, and importing modules from the standard library.

Core rules

• open(path, mode) returns a file object. Modes: "r" read (default, file must exist), "w" write (creates or truncates), "a" append, "r+" read/write, plus "b" for binary and "t" for text.
• Use a context manager: with open("data.txt") as f: closes the file automatically, even if an exception occurs.
• Reading: f.read() returns the whole file as one string; f.readline() returns one line including \n; f.readlines() returns a list of lines. Iterating over f yields lines.
• Writing: f.write(s) writes a string and returns the character count; f.writelines(seq) writes an iterable of strings. No newline is added automatically.
• Import forms: import math then math.sqrt(9); from math import sqrt then sqrt(9); import numpy as np aliases. Importing executes the module's top-level code once, then caches it in sys.modules.

Worked example

with open("out.txt", "w") as f: f.write("a\nb") with open("out.txt") as f: print(f.read().splitlines()) Output: ['a', 'b']. The first with creates or truncates the file and writes two lines; the second opens for reading and read() returns "a\nb", which splitlines() turns into a list.

Traps

• "w" erases existing content; use "a" to preserve it.
• f.write(5) raises TypeError; convert with str(5).
• Reading twice returns "" the second time because the position is at end-of-file; use f.seek(0) to rewind.
• from math import * pollutes the namespace and can hide names.
• Importing a missing module raises ModuleNotFoundError.