Hướng dẫn error check python
Until now error messages haven’t been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions. Show 8.1. Syntax Errors¶Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python: >>> while True print('Hello world') File " The parser repeats the offending line and displays a little ‘arrow’ 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 function 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 " The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are The rest of the line provides detail based on the type of exception and what caused it. The preceding part of the error message shows the context where the exception occurred, in the form of a stack traceback. In general it contains a stack traceback listing source lines; however, it will not display lines read from standard input. Built-in Exceptions lists the built-in exceptions and their meanings. 8.3. Handling Exceptions¶It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using Control-C or whatever the operating
system supports); note that a user-generated interruption is signalled by raising the >>> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ... The
A ... except (RuntimeError, TypeError, NameError): ... pass A class in an class B(Exception): pass class C(B): pass class D(C): pass for cls in [B, C, D]: try: raise cls() except D: print("D") except C: print("C") except B: print("B") Note that if the except clauses were reversed (with All exceptions inherit from
import sys try: f = open('myfile.txt') s = f.readline() i = int(s.strip()) except OSError as err: print("OS error: {0}".format(err)) except ValueError: print("Could not convert data to an integer.") except BaseException as err: print(f"Unexpected {err=}, {type(err)=}") raise Alternatively the last except clause may omit the exception name(s), however
the exception value must then be retrieved from The for arg in sys.argv[1:]: try: f = open(arg, 'r') except OSError: print('cannot open', arg) else: print(arg, 'has', len(f.readlines()), 'lines') f.close() The use of the When an exception occurs, it may have an associated value, also known as the exception’s argument. The presence and type of the argument depend on the exception type. The except clause may specify a variable after the exception name. The variable is bound to an exception instance with the arguments stored in >>> try: ... raise Exception('spam', 'eggs') ... except Exception as inst: ... print(type(inst)) # the exception instance ... print(inst.args) # arguments stored in .args ... print(inst) # __str__ allows args to be printed directly, ... # but may be overridden in exception subclasses ... x, y = inst.args # unpack args ... print('x =', x) ... print('y =', y) ... If an exception has arguments, they are printed as the last part (‘detail’) of the message for unhandled exceptions. Exception handlers don’t just handle exceptions if they occur immediately in the try clause, but also if they occur inside functions that are called (even indirectly) in the try clause. For example: >>> def this_fails(): ... x = 1/0 ... >>> try: ... this_fails() ... except ZeroDivisionError as err: ... print('Handling run-time error:', err) ... Handling run-time error: division by zero 8.4. Raising Exceptions¶The >>> raise NameError('HiThere') Traceback (most recent call last): File " The sole argument to raise ValueError # shorthand for 'raise ValueError()' If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the
>>> try: ... raise NameError('HiThere') ... except NameError: ... print('An exception flew by!') ... raise ... An exception flew by! Traceback (most recent call last): File " 8.5. Exception Chaining¶The
# exc must be exception instance or None. raise RuntimeError from exc This can be useful when you are transforming exceptions. For example: >>> def func(): ... raise ConnectionError ... >>> try: ... func() ... except ConnectionError as exc: ... raise RuntimeError('Failed to open database') from exc ... Traceback (most recent call last): File " Exception chaining happens automatically when an exception is raised inside an
>>> try: ... open('database.sqlite') ... except OSError: ... raise RuntimeError from None ... Traceback (most recent call last): File " For more information about chaining mechanics, see Built-in Exceptions. 8.6. User-defined Exceptions¶Programs may name their own exceptions by creating a new exception class (see Classes for more about Python classes). Exceptions should
typically be derived from the Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception. Most exceptions are defined with names that end in “Error”, similar to the naming of the standard exceptions. Many standard modules define their own exceptions to report errors that may occur in functions they define. More information on classes is presented in chapter Classes. 8.7. Defining Clean-up Actions¶The >>> try: ... raise KeyboardInterrupt ... finally: ... print('Goodbye, world!') ... Goodbye, world! KeyboardInterrupt Traceback (most recent call last): File " If a
For example: >>> def bool_return(): ... try: ... return True ... finally: ... return False ... >>> bool_return() False A more complicated example: >>> def divide(x, y): ... try: ... result = x / y ... except ZeroDivisionError: ... print("division by zero!") ... else: ... print("result is", result) ... finally: ... print("executing finally clause") ... >>> divide(2, 1) result is 2.0 executing finally clause >>> divide(2, 0) division by zero! executing finally clause >>> divide("2", "1") executing finally clause Traceback (most recent call last): File " As you can see, the In real world applications, the
8.8. Predefined Clean-up Actions¶Some objects define standard clean-up actions to be undertaken when the object is no longer needed, regardless of whether or not the operation using the object succeeded or failed. Look at the following example, which tries to open a file and print its contents to the screen. for line in open("myfile.txt"): print(line, end="") The problem with this code is that it
leaves the file open for an indeterminate amount of time after this part of the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. The with open("myfile.txt") as f: for line in f: print(line, end="") After the statement is executed, the file f is always closed, even if a problem was encountered while processing the lines. Objects which, like files, provide predefined clean-up actions will indicate this in their documentation. |