8. 错误和异常¶
到目前为止,我们还没有提到错误消息,但是如果你已经尝试过这些例子,你可能已经看过了一些错误消息。 目前有(至少)两种可区分的错误:语法错误*和*异常。
8.1. 语法错误¶
语法错误又称代码解析错误,是Python初学者最容易遇到的错误。
>>> while True print('Hello world')
File "<stdin>", line 1
while True print('Hello world')
^
SyntaxError: invalid syntax
解析器会输出出现语法错误的那一行,并显示一个“箭头”,指向这行里面检测到第一个错误。 错误是由箭头指示的位置的*前面*的标识符引起的,也有可能是在这里才检测除了。:在示例中,在func:print`这个函数中检测到错误:,因为少了个冒号 (`’:’``)。还会输出文件名和行号,以便输入来自脚本文件时知道错误在哪里。
8.2. 异常¶
即使语句或表达式在语法上是正确的,但在尝试执行它时可能会导致错误。 在执行期间检测到的错误被称为*异常*,异常不一定会导致严重后果:您将很快学会如何在Python程序中处理它们。 但是,大多数异常并不会被程序处理,此时会显示如下所示的错误信息:
>>> 10 * (1/0)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> 4 + spam*3
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly
错误信息的最后一行告诉我们程序遇到了什么类型的错误。异常有不同的类型,而其类型名称将会在错误信息中输出出来。上述样例中的异常类型依次是:ZeroDivisionError
, NameError
and TypeError
。错误信息中的异常类型是执行时抛出的内建异常类型。所有的内建类型都会如此,而虽然这是惯例,但是用户自定义的异常不一定如此。标准的异常类型是内建的标识符而非保留关键字。
这一行剩下的部分能告诉我们此处抛出异常的具体类型以及哪一部分代码抛出了异常。
The preceding part of the error message shows the context where the exception happened, 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 KeyboardInterrupt
exception.
>>> while True:
... try:
... x = int(input("Please enter a number: "))
... break
... except ValueError:
... print("Oops! That was no valid number. Try again...")
...
The try
statement works as follows.
- First, the try clause (the statement(s) between the
try
andexcept
keywords) is executed. - If no exception occurs, the except clause is skipped and execution of the
try
statement is finished. - If an exception occurs during execution of the try clause, the rest of the
clause is skipped. Then if its type matches the exception named after the
except
keyword, the except clause is executed, and then execution continues after thetry
statement. - If an exception occurs which does not match the exception named in the except
clause, it is passed on to outer
try
statements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above.
A try
statement may have more than one except clause, to specify
handlers for different exceptions. At most one handler will be executed.
Handlers only handle exceptions that occur in the corresponding try clause, not
in other handlers of the same try
statement. An except clause may
name multiple exceptions as a parenthesized tuple, for example:
... except (RuntimeError, TypeError, NameError):
... pass
A class in an except
clause is compatible with an exception if it is
the same class or a base class thereof (but not the other way around — an
except clause listing a derived class is not compatible with a base class). For
example, the following code will print B, C, D in that order:
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 except B
first), it
would have printed B, B, B — the first matching except clause is triggered.
The last except clause may omit the exception name(s), to serve as a wildcard. Use this with extreme caution, since it is easy to mask a real programming error in this way! It can also be used to print an error message and then re-raise the exception (allowing a caller to handle the exception as well):
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:
print("Unexpected error:", sys.exc_info()[0])
raise
The try
… except
statement has an optional else
clause, which, when present, must follow all except clauses. It is useful for
code that must be executed if the try clause does not raise an exception. For
example:
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 else
clause is better than adding additional code to
the try
clause because it avoids accidentally catching an exception
that wasn’t raised by the code being protected by the try
…
except
statement.
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
instance.args
. For convenience, the exception instance defines
__str__()
so the arguments can be printed directly without having to
reference .args
. One may also instantiate an exception first before
raising it and add any attributes to it as desired.
>>> 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)
...
<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs
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
statement allows the programmer to force a specified
exception to occur. For example:
>>> raise NameError('HiThere')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: HiThere
The sole argument to raise
indicates the exception to be raised.
This must be either an exception instance or an exception class (a class that
derives from Exception
). If an exception class is passed, it will
be implicitly instantiated by calling its constructor with no arguments:
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 raise
statement allows you to
re-raise the exception:
>>> try:
... raise NameError('HiThere')
... except NameError:
... print('An exception flew by!')
... raise
...
An exception flew by!
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
NameError: HiThere
8.5. User-defined Exceptions¶
Programs may name their own exceptions by creating a new exception class (see
类 for more about Python classes). Exceptions should typically
be derived from the Exception
class, either directly or indirectly.
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. When creating a module that can raise several distinct errors, a common practice is to create a base class for exceptions defined by that module, and subclass that to create specific exception classes for different error conditions:
class Error(Exception):
"""Base class for exceptions in this module."""
pass
class InputError(Error):
"""Exception raised for errors in the input.
Attributes:
expression -- input expression in which the error occurred
message -- explanation of the error
"""
def __init__(self, expression, message):
self.expression = expression
self.message = message
class TransitionError(Error):
"""Raised when an operation attempts a state transition that's not
allowed.
Attributes:
previous -- state at beginning of transition
next -- attempted new state
message -- explanation of why the specific transition is not allowed
"""
def __init__(self, previous, next, message):
self.previous = previous
self.next = next
self.message = message
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 类.
8.6. Defining Clean-up Actions¶
The try
statement has another optional clause which is intended to
define clean-up actions that must be executed under all circumstances. For
example:
>>> try:
... raise KeyboardInterrupt
... finally:
... print('Goodbye, world!')
...
Goodbye, world!
KeyboardInterrupt
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
A finally clause is always executed before leaving the try
statement, whether an exception has occurred or not. When an exception has
occurred in the try
clause and has not been handled by an
except
clause (or it has occurred in an except
or
else
clause), it is re-raised after the finally
clause has
been executed. The finally
clause is also executed “on the way out”
when any other clause of the try
statement is left via a
break
, continue
or return
statement. 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 "<stdin>", line 1, in <module>
File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'
As you can see, the finally
clause is executed in any event. The
TypeError
raised by dividing two strings is not handled by the
except
clause and therefore re-raised after the finally
clause has been executed.
In real world applications, the finally
clause is useful for
releasing external resources (such as files or network connections), regardless
of whether the use of the resource was successful.
8.7. 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
statement allows objects like files to be
used in a way that ensures they are always cleaned up promptly and correctly.
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.