Reading File Contents
The failure happens while the program is running. It is not a syntax mistake in the program text. Python reaches the file-opening operation, cannot find the requested file, and reports the runtime problem.
When the File Is Missing
A program may try to open a file that has been deleted, moved, renamed, or never created. When open() targets a file that does not exist, Python cannot obtain a file handle. Instead, it raises a FileNotFoundError.
The failure happens while the program is running. It is not a syntax mistake in the program text. Python reaches the file-opening operation, cannot find the requested file, and reports the runtime problem.
What do you think happens?
Suppose a program tries to open a missing file and has no error handling. What happens to the statements written after the failed open() call?
Reveal answer
They are not reached because execution stops
Without error handling, FileNotFoundError stops program execution immediately. The program displays a traceback, and subsequent code does not run.
The Failed Execution Path
The important point in this flow is that the failed open() call does not produce a usable file handle. Because the error is not caught, Python does not continue to the statements that follow the call. It displays the traceback and terminates the program.
Tracing a Missing File
A program attempts to open a file that is not present and has no try and except statements. Predict the result.
- 1Attempt the operation
The program reaches
open()and asks Python to access the requested file. - 2Check the result
The file cannot be found, so Python raises
FileNotFoundErrorinstead of providing a file handle. - 3Follow execution
Because no matching error handler is present, execution stops at the failed operation. Statements after it are not run.
- 4Read the report
Python displays a traceback describing where the failure occurred and identifying the error as
FileNotFoundError.
The program terminates with a FileNotFoundError traceback and does not execute the code after the failed open() call.
Reading the Traceback
A traceback is an error report that helps you locate the failure. Its first line indicates that a traceback follows. The next information identifies the file and line number where the problem occurred. Another line shows the exact code that failed. The final line names the error type, FileNotFoundError, and gives details about the file that could not be found.
FileNotFoundError is Python's indication that it looked for the requested file but could not find it at the location specified by the program.
Redirecting Control with try and except
A try block contains an operation that might fail. An except block provides the response for a matching error. When open() inside the try block raises FileNotFoundError, Python leaves the remaining statements in that try block and jumps to the matching except block. After the except block finishes, execution continues normally.
Tracing the Handled Path
A file-opening operation is placed inside a try block, followed by an except block that handles FileNotFoundError. Predict the execution order when the file is missing.
- 1Enter try
Python begins executing the statements in the try block.
- 2Raise the error
The missing file causes
open()to raiseFileNotFoundError, so no file handle is obtained. - 3Jump to except
Python transfers control to the matching except block rather than terminating immediately with an unhandled traceback.
- 4Finish handling
The statements in the except block run as the program's response to the missing file.
- 5Continue
After the except block completes, execution continues normally.
The failure is caught, the except block handles it, and the program can continue instead of stopping with an unhandled traceback.
Place the file-opening operation that may fail inside a try block and provide a specific except response for FileNotFoundError. A sensible response can inform the user, request a different filename, use a default file, or take another appropriate action.
Two Execution Outcomes
| Without error handling | With try and except |
|---|---|
| open() raises FileNotFoundError | open() raises FileNotFoundError |
| No matching handler receives the error | The matching except block receives the error |
| A traceback is displayed | The program can provide a controlled response |
| Execution stops immediately | Execution continues after the except block completes |
| Code after the failed call is not reached | Later code can be reached after handling |
The missing file causes the same error in both cases; error handling changes what the program does next.
The try and except structure does not make the missing file appear and does not create a file handle. It changes the control flow after the failure. Without a handler, the traceback reaches the user and the program terminates. With a matching handler, the program responds deliberately and can continue.
Mistakes to Avoid
Common mistake Assuming open() always produces a file handle
What it looks likeThe requested file has been moved or deleted.
Why it failsA missing file causes
FileNotFoundError, so Python cannot provide a file handle.Do this insteadTreat file opening as an operation that may fail and handle FileNotFoundError when appropriate.
Common mistake Expecting statements after an unhandled failure to run
What it looks likeThe program tries to open a missing file and then expects later work to continue.
Why it failsWithout error handling, the error stops execution immediately.
Do this insteadPut the operation in a try block and define the response in a matching except block.
Common mistake Reading only the final error name
What it looks likeSeeing FileNotFoundError but ignoring the file and line information above it.
Why it failsThe traceback also identifies where the failure occurred and shows the exact code that failed.
Do this insteadRead the traceback from its location information through its final error message.
Common mistake Treating the traceback as a syntax error
What it looks likeThe file-opening statement is valid program syntax, but the requested file is unavailable.
Why it failsThis is a runtime error caused by the file not being where the program expects it.
Do this insteadDistinguish the validity of the statement from the availability of the file during execution.
Practice the Control Flow
A program attempts to open a file that does not exist. It has a try block containing the opening operation, a matching except block, and statements after the except block. Describe the order in which the try block, failed opening operation, except block, and later statements execute. Then describe how the order changes if the except block is removed.
Hints
- A missing file causes
FileNotFoundErrorbefore a file handle is obtained. - An error raised in a try block transfers control to the matching except block.
- Without a matching handler, execution stops and later statements are not reached.
To check your reasoning, the handled path is: enter try, fail at open(), jump to except, finish the response, then continue. The unhandled path is: enter open(), raise FileNotFoundError, display the traceback, and stop.
Reliable File Access
Files depend on conditions outside the program's immediate control. A file may be deleted, moved, renamed, or never created. Preparing for FileNotFoundError therefore makes file-related software more robust. Instead of exposing an unexpected traceback and stopping, the program can communicate the problem and choose a sensible next action.
Key Takeaways
- Calling
open()for a file that does not exist raisesFileNotFoundErrorand does not produce a file handle. - Without error handling, the traceback identifies the failure and program execution stops immediately.
- A traceback shows that a traceback is coming, where the error occurred, the code that failed, and the
FileNotFoundErrordetails. - A try block holds the operation that may fail, while a matching except block handles
FileNotFoundError. - After the except block completes, execution can continue normally, allowing the program to respond to the missing file gracefully.