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Errors & Files — Cheat Sheet

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Errors & Files

Python A–Z4 topicsQuick revision reference

Exceptions — try/except/else/finally

Python handles failures with exceptions: catch specific types with except, run success-only code in else, guarantee cleanup in finally — and never write a bare except.

✓Catch specific exceptions; bare except: also swallows Ctrl+C — never use it
✓else runs only when the try block succeeded; finally always runs
✓EAFP (try/except) is idiomatic over pre-checking (LBYL)
✓ValueError = bad value, TypeError = wrong type — interviewers check you know the difference

try / except / else / finally — each part's job

def read_marks(path):
    try:
        f = open(path)                      # may raise FileNotFoundError
        marks = [int(line) for line in f]   # may raise ValueError
    except FileNotFoundError:
        print("file missing — using empty list")
        return []
    except ValueError as e:
        print(f"bad number in file: {e}")
        return []
    else:
        print(f"loaded {len(marks)} marks")  # ONLY if no exception
        return marks
    finally:
        try: f.close()                       # ALWAYS runs
        except NameError: pass               # open itself failed

# Hierarchy matters — order except blocks specific -> general:
# except ZeroDivisionError: ...     (child of ArithmeticError)
# except ArithmeticError: ...       (child of Exception)
# A bare 'except:' also catches KeyboardInterrupt/SystemExit — never use it.

Raising, Custom Exceptions & Chaining

raise signals failure; custom exception classes give your domain a vocabulary; raise...from chains causes so tracebacks tell the whole story.

✓Define a base exception per domain; subclass for specific failures
✓raise NewError(...) from original preserves the causal chain in tracebacks
✓Bare raise inside except re-raises the active exception (log-and-propagate)
✓Inherit from Exception, not BaseException (that would break Ctrl+C)

A domain vocabulary for failures

class PaymentError(Exception):
    """Base for all payment failures."""

class CardDeclined(PaymentError):
    def __init__(self, card_last4, reason):
        super().__init__(f"card *{card_last4} declined: {reason}")
        self.card_last4 = card_last4
        self.reason = reason

class GatewayTimeout(PaymentError):
    pass

def charge(card, amount):
    if amount > 50_000:
        raise CardDeclined(card[-4:], "limit exceeded")
    return "ok"

try:
    charge("4242424242424242", 99_999)
except CardDeclined as e:
    print("specific:", e.reason)        # handle precisely
except PaymentError:
    print("some other payment issue")   # catch-all for the domain

Files & the with Statement

open() + with reads and writes files with guaranteed closing; iterate file objects line by line for constant-memory processing of files of any size.

✓Always with open(...) — guaranteed close on success AND exception
✓Iterate the file object for constant memory; read() only for small files
✓Mode "w" truncates at open; "a" appends; add encoding="utf-8" for text
✓write() adds no newline; print(..., file=f) does

with + line iteration — the only pattern you need

# students.txt: one "name,marks" per line
with open("students.txt", encoding="utf-8") as f:
    for line in f:                      # streams — file can be 10 GB
        name, marks = line.strip().split(",")
        print(name, int(marks))
# file auto-closed here, even on exceptions

# Small files — grab it all
with open("config.txt", encoding="utf-8") as f:
    text = f.read()

# Multiple files in one with
with open("in.txt") as src, open("out.txt", "w") as dst:
    for line in src:
        dst.write(line.upper())

# Without with (what NOT to do):
# f = open("x.txt"); data = f.read(); f.close()
#  ^ an exception between open and close leaks the handle

JSON, CSV & pathlib

json.load/dump round-trip Python dicts to the web's data format, csv handles tabular files safely, and pathlib replaces string paths with a clean object API.

✓json: loads/dumps for strings, load/dump for files; keys become strings, tuples become lists
✓Use csv module (DictReader/DictWriter) — never split(",") manually
✓pathlib.Path joins with /, globs, mkdirs, and read_text/write_text in one call
✓Always newline="" when opening CSV files (per the csv docs) and encoding="utf-8"

loads/dumps (strings) vs load/dump (files)

import json

profile = {"name": "Asha", "skills": ["python", "sql"], "cgpa": 8.7}

s = json.dumps(profile, indent=2)     # dict -> pretty string
print(s)

back = json.loads(s)                  # string -> dict
print(back["skills"][0])              # python

with open("profile.json", "w") as f:  # dict -> file
    json.dump(profile, f, indent=2)

with open("profile.json") as f:       # file -> dict
    data = json.load(f)

# Gotchas
json.dumps({1: "a"})       # keys stringified: '{"1": "a"}'
json.dumps((1, 2))         # tuples become lists: '[1, 2]'
# json.dumps(datetime.now())  -> TypeError: use .isoformat() first

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Errors & Files

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Errors & Files

Exceptions — try/except/else/finally

✓Catch specific exceptions; bare except: also swallows Ctrl+C — never use it
✓else runs only when the try block succeeded; finally always runs
✓EAFP (try/except) is idiomatic over pre-checking (LBYL)
✓ValueError = bad value, TypeError = wrong type — interviewers check you know the difference

def read_marks(path):
    try:
        f = open(path)                      # may raise FileNotFoundError
        marks = [int(line) for line in f]   # may raise ValueError
    except FileNotFoundError:
        print("file missing — using empty list")
        return []
    except ValueError as e:
        print(f"bad number in file: {e}")
…

AiCanCode.org

Errors & Files

Raising, Custom Exceptions & Chaining

✓Define a base exception per domain; subclass for specific failures
✓raise NewError(...) from original preserves the causal chain in tracebacks
✓Bare raise inside except re-raises the active exception (log-and-propagate)
✓Inherit from Exception, not BaseException (that would break Ctrl+C)

class PaymentError(Exception):
    """Base for all payment failures."""

class CardDeclined(PaymentError):
    def __init__(self, card_last4, reason):
        super().__init__(f"card *{card_last4} declined: {reason}")
        self.card_last4 = card_last4
        self.reason = reason

…

AiCanCode.org

Errors & Files

Files & the with Statement

✓Always with open(...) — guaranteed close on success AND exception
✓Iterate the file object for constant memory; read() only for small files
✓Mode "w" truncates at open; "a" appends; add encoding="utf-8" for text
✓write() adds no newline; print(..., file=f) does

# students.txt: one "name,marks" per line
with open("students.txt", encoding="utf-8") as f:
    for line in f:                      # streams — file can be 10 GB
        name, marks = line.strip().split(",")
        print(name, int(marks))
# file auto-closed here, even on exceptions

# Small files — grab it all
with open("config.txt", encoding="utf-8") as f:
…

AiCanCode.org

Errors & Files

JSON, CSV & pathlib

✓json: loads/dumps for strings, load/dump for files; keys become strings, tuples become lists
✓Use csv module (DictReader/DictWriter) — never split(",") manually
✓pathlib.Path joins with /, globs, mkdirs, and read_text/write_text in one call
✓Always newline="" when opening CSV files (per the csv docs) and encoding="utf-8"

import json

profile = {"name": "Asha", "skills": ["python", "sql"], "cgpa": 8.7}

s = json.dumps(profile, indent=2)     # dict -> pretty string
print(s)

back = json.loads(s)                  # string -> dict
print(back["skills"][0])              # python
…

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