Lỗi invalid syntax trong else trong python 3 năm 2024

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.

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 "", line 1

while True print('Hello world')
               ^

SyntaxError: invalid syntax

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 , since a colon (

> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ...

7. is missing before it. File name and line number are printed 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 "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

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 , and . The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords).

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.

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 exception.

> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ...

The statement works as follows.

• First, the try clause (the statement(s) between the and keywords) is executed.
• If no exception occurs, the except clause is skipped and execution of the statement is finished.
• If an exception occurs during execution of the clause, the rest of the clause is skipped. Then, if its type matches the exception named after the keyword, the except clause is executed, and then execution continues after the try/except block.
• If an exception occurs which does not match the exception named in the except clause, it is passed on to outer statements; if no handler is found, it is an unhandled exception and execution stops with an error message.

A 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

... except (RuntimeError, TypeError, NameError): ... pass

2 statement. An except clause may name multiple exceptions as a parenthesized tuple, for example:

... except (RuntimeError, TypeError, NameError): ... pass

A class in an 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

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")

2 first), it would have printed B, B, B — the first matching except clause is triggered.

When an exception occurs, it may have associated values, also known as the exception’s arguments. The presence and types of the arguments depend on the exception type.

The except clause may specify a variable after the exception name. The variable is bound to the exception instance which typically has 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")

3 attribute that stores the arguments. For convenience, builtin exception types define to print all the arguments without explicitly accessing

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")

5.

> try: ... raise Exception('spam', 'eggs') ... except Exception as inst: ... print(type(inst)) # the exception type ... 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) ... ('spam', 'eggs') ('spam', 'eggs') x = spam y = eggs

The exception’s output is printed as the last part (‘detail’) of the message for unhandled exceptions.

is the common base class of all exceptions. One of its subclasses, , is the base class of all the non-fatal exceptions. Exceptions which are not subclasses of are not typically handled, because they are used to indicate that the program should terminate. They include which is raised by and which is raised when a user wishes to interrupt the program.

can be used as a wildcard that catches (almost) everything. However, it is good practice to be as specific as possible with the types of exceptions that we intend to handle, and to allow any unexpected exceptions to propagate on.

The most common pattern for handling is to print or log the exception and then re-raise it (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:", err)

except ValueError:

print("Could not convert data to an integer.")

except Exception as err:

print(f"Unexpected {err=}, {type(err)=}")
raise

The … 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

> try: ... raise Exception('spam', 'eggs') ... except Exception as inst: ... print(type(inst)) # the exception type ... 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) ... ('spam', 'eggs') ('spam', 'eggs') x = spam y = eggs

7 clause is better than adding additional code to the clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the

... except (RuntimeError, TypeError, NameError): ... pass

2 …

... except (RuntimeError, TypeError, NameError): ... pass

4 statement.

Exception handlers do not handle only exceptions that occur immediately in the try clause, but also those that 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 statement allows the programmer to force a specified exception to occur. For example:

> raise NameError('HiThere') Traceback (most recent call last): File "", line 1, in NameError: HiThere

The sole argument to indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from , such as or one of its subclasses). If an exception class is passed, it will be implicitly instantiated by calling its constructor with no arguments:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

0

If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the statement allows you to re-raise the exception:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

1

8.5. Exception Chaining

If an unhandled exception occurs inside an section, it will have the exception being handled attached to it and included in the error message:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

2

To indicate that an exception is a direct consequence of another, the statement allows an optional clause:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

3

This can be useful when you are transforming exceptions. For example:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

4

It also allows disabling automatic exception chaining using the

import sys try:

f = open('myfile.txt')
s = f.readline()
i = int(s.strip())

except OSError as err:

print("OS error:", err)

except ValueError:

print("Could not convert data to an integer.")

except Exception as err:

print(f"Unexpected {err=}, {type(err)=}")
raise

9 idiom:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

5

For more information about chaining mechanics, see .

8.6. User-defined Exceptions

Programs may name their own exceptions by creating a new exception class (see for more about Python classes). Exceptions should typically be derived from the 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.

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.

8.7. Defining Clean-up Actions

The statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

6

If a clause is present, 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()

2 clause will execute as the last task before the statement completes. 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()

2 clause runs whether or not the

... except (RuntimeError, TypeError, NameError): ... pass

2 statement produces an exception. The following points discuss more complex cases when an exception occurs:

• If an exception occurs during execution of the ... except (RuntimeError, TypeError, NameError): ... pass 2 clause, the exception may be handled by an clause. If the exception is not handled by an ... except (RuntimeError, TypeError, NameError): ... pass 4 clause, the exception is re-raised after 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()  
  

  2 clause has been executed.
• An exception could occur during execution of an ... except (RuntimeError, TypeError, NameError): ... pass 4 or

  > try: ... raise Exception('spam', 'eggs') ... except Exception as inst: ... print(type(inst)) # the exception type ... 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) ... ('spam', 'eggs') ('spam', 'eggs') x = spam y = eggs 7 clause. Again, the exception is re-raised after 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()  
  

  2 clause has been executed.

  If 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()  
  

  2 clause executes a , or statement, exceptions are not re-raised.

  If the ... except (RuntimeError, TypeError, NameError): ... pass 2 statement reaches a , or statement, 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()  
  

  2 clause will execute just prior to the

  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 5, 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 6 or 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 7 statement’s execution.

• If a 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()  
  

  2 clause includes a 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 7 statement, the returned value will be the one 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()  
  

  2 clause’s 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 7 statement, not the value from the ... except (RuntimeError, TypeError, NameError): ... pass 2 clause’s 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 7 statement.

For example:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

7

A more complicated example:

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

8

As you can see, the clause is executed in any event. The raised by dividing two strings is not handled by the clause and therefore re-raised after 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()

2 clause has been executed.

In real world applications, the clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.

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.

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

9

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 statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.

> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ...

0

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.

8.9. Raising and Handling Multiple Unrelated Exceptions

There are situations where it is necessary to report several exceptions that have occurred. This is often the case in concurrency frameworks, when several tasks may have failed in parallel, but there are also other use cases where it is desirable to continue execution and collect multiple errors rather than raise the first exception.

The builtin wraps a list of exception instances so that they can be raised together. It is an exception itself, so it can be caught like any other exception.

> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ...

1

By using

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

09 instead of

... except (RuntimeError, TypeError, NameError): ... pass

4, we can selectively handle only the exceptions in the group that match a certain type. In the following example, which shows a nested exception group, each

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

09 clause extracts from the group exceptions of a certain type while letting all other exceptions propagate to other clauses and eventually to be reraised.

> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ...

2

Note that the exceptions nested in an exception group must be instances, not types. This is because in practice the exceptions would typically be ones that have already been raised and caught by the program, along the following pattern:

> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ...

3

8.10. Enriching Exceptions with Notes

When an exception is created in order to be raised, it is usually initialized with information that describes the error that has occurred. There are cases where it is useful to add information after the exception was caught. For this purpose, exceptions have a method

> 10 * (1/0) Traceback (most recent call last): File "", line 1, in ZeroDivisionError: division by zero

4 + spam*3 Traceback (most recent call last): File "", line 1, in NameError: name 'spam' is not defined

'2' + 2 Traceback (most recent call last): File "", line 1, in TypeError: can only concatenate str (not "int") to str

12 that accepts a string and adds it to the exception’s notes list. The standard traceback rendering includes all notes, in the order they were added, after the exception.

> while True: ... try: ... x = int(input("Please enter a number: ")) ... break ... except ValueError: ... print("Oops! That was no valid number. Try again...") ...

4

For example, when collecting exceptions into an exception group, we may want to add context information for the individual errors. In the following each exception in the group has a note indicating when this error has occurred.