68 — Errors: handling problems

A missing file or a line that is not a number stops a program dead. try / except catches it and runs a friendly fallback.

#Steps

1. A missing file makes file_read raise FileNotFoundError; the except block runs instead of crashing:

   use file latest
   try:
       text = file_read("notes.txt")
   except FileNotFoundError:
       show "notes.txt is not here yet."
   

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2. int() raises ValueError on bad input; else after except runs only when nothing went wrong:

   ask answer, "A number to double: "
   try:
       number = int(answer)
   except ValueError:
       show "That is not a number."
   else:
       show f"Doubled, it is {number * 2}."
   

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3. The full program reads a file that may be missing, then asks for a number until the answer converts. Create notes.txt with two lines first:

   # safe_read.nme — read a file that may be missing, safely.
   # Run: nme r safe_read
   # try / except keeps the program alive when the data is bad.
   
   use file latest
   
   ask file_name, "File to read (for example notes.txt): "
   
   try:
       text = file_read(file_name)
   except FileNotFoundError:
       show "That file is not here yet."
       show "Create it first with file_write, then run again."
   else:
       show f"Read {len(text)} characters."
       show f"That is {len(text.split())} words."
       lines = text.splitlines()
       show f"The first line is: {lines[0]}"
       show "Contents:"
       show text
   
   show ""
   show "Now give me a number to double."
   while True:
       ask answer, "A number to double: "
       try:
           number = int(answer)
       except ValueError:
           show f"'{answer}' is not a number — try again."
       else:
           break
   
   show f"Doubled, it is {number * 2}."
   

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4. Run it with the file present:

   printf 'notes.txt\nseven\n12\n' | nme r safe_read
   

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   File to read (for example notes.txt): Read 30 characters.
   That is 6 words.
   The first line is: Today is sunny.
   Contents:
   Today is sunny.
   We study NME.
   
   Now give me a number to double.
   A number to double: 'seven' is not a number — try again.
   A number to double: Doubled, it is 24.
   

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   With a file name that does not exist:

   printf 'nope.txt\n12\n' | nme r safe_read
   

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   File to read (for example notes.txt): That file is not here yet.
   Create it first with file_write, then run again.
   
   Now give me a number to double.
   A number to double: Doubled, it is 24.
   

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5. Catch named errors (FileNotFoundError, ValueError), not a bare except:, so unexpected bugs still crash loudly.

#Try it yourself

Read a JSON file with json_load and catch json.JSONDecodeError, or change the number loop to add two numbers.

#What you learned

• try: runs risky code; except SomeError: catches exactly that error.
• FileNotFoundError means a missing file; ValueError means int() could not convert.
• else: after except runs only when no error happened.
• Catch named errors so unexpected bugs still crash loudly.