File Error Python
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you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions. 8.1. python exception message Syntax Errors¶ Syntax errors, also known as parsing errors, are perhaps
Python Custom Exception
the most common kind of complaint you get while you are still learning Python: >>> while True
Python Exception Stack Trace
print 'Hello world' File "
Python Raise Valueerror
pointing at the earliest point in the line where the error was detected. The error is caused by (or at least detected at) the token preceding the arrow: in the example, the error is detected at the keyword print, since a colon (':') is missing before it. File name and line number are printed syntax for generic except clause in python so you know where to look in case the input came from a script. 8.2. Exceptions¶ Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here: >>> 10 * (1/0) Traceback (most recent call last): File "
This module never needs to be imported explicitly: the exceptions are provided in the built-in namespace as well as the exceptions module. For class python print exception exceptions, in a try statement with an except clause is nested try block possible in python that mentions a particular class, that clause also handles any exception classes derived from that class (but not python try except else exception classes from which it is derived). Two exception classes that are not related via subclassing are never equivalent, even if they have the same name. The built-in exceptions https://docs.python.org/2/tutorial/errors.html listed below can be generated by the interpreter or built-in functions. Except where mentioned, they have an "associated value" indicating the detailed cause of the error. This may be a string or a tuple containing several items of information (e.g., an error code and a string explaining the code). The associated value is the second argument to the https://docs.python.org/2/library/exceptions.html class="pre">raise statement. If the exception class is derived from the standard root class BaseException, the associated value is present as the exception instance's args attribute. User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition "just like" the situation in which the interpreter raises the same exception; but beware that there is nothing to prevent user code from raising an inappropriate error. The built-in exception classes can be subclassed to define new exceptions; programmers are encouraged to derive new exceptions from the Exception class or one of its subclasses, and not from BaseException. More information on defining exceptions is available in the Python Tutorial under User-defined Exceptions. The following exceptions are only used as base classes for other exceptions. exception BaseException¶ The base class for all built-in exceptions. It is not meant to be directly inherited by user-defined classes (for that, use Exception). If str() or unicode() is called on an instance of this class, the representa
Working with File Objects 6.2.1. Reading Files 6.2.2. Closing Files 6.2.3. Handling I/O Errors 6.2.4. Writing to Files 6.3. Iterating with for Loops 6.4. Using sys.modules 6.5. Working with Directories 6.6. Putting It All Together 6.7. Summary In this http://www.diveintopython.net/file_handling/ chapter, you will dive into exceptions, file objects, for loops, and the os and sys modules. If you've used exceptions in another programming language, you can skim the first section to get a sense of Python's syntax. Be sure to tune in again for file handling. 6.1.Handling Exceptions 6.1.1. Using Exceptions For Other Purposes Like many other programming languages, Python has exception handling via try...except blocks. Python uses try...except to handle exceptions python exception and raise to generate them. Java and C++ use try...catch to handle exceptions, and throw to generate them. Exceptions are everywhere in Python. Virtually every module in the standard Python library uses them, and Python itself will raise them in a lot of different circumstances. You've already seen them repeatedly throughout this book. Accessing a non-existent dictionary key will raise a KeyError exception. Searching a list for a non-existent value will raise a file error python ValueError exception. Calling a non-existent method will raise an AttributeError exception. Referencing a non-existent variable will raise a NameError exception. Mixing datatypes without coercion will raise a TypeError exception. In each of these cases, you were simply playing around in the Python IDE: an error occurred, the exception was printed (depending on your IDE, perhaps in an intentionally jarring shade of red), and that was that. This is called an unhandled exception. When the exception was raised, there was no code to explicitly notice it and deal with it, so it bubbled its way back to the default behavior built in to Python, which is to spit out some debugging information and give up. In the IDE, that's no big deal, but if that happened while your actual Python program was running, the entire program would come to a screeching halt. An exception doesn't need result in a complete program crash, though. Exceptions, when raised, can be handled. Sometimes an exception is really because you have a bug in your code (like accessing a variable that doesn't exist), but many times, an exception is something you can anticipate. If you're opening a file, it might not exist. If you're connecting to a database, it might be unavailable, or you might not have the correct security credentials