AttributeError: __enter__ exception in Python [Solved]

# AttributeError: enter exception in Python [Solved]

The Python "AttributeError: __enter__" exception occurs for multiple reasons:

Using a string as a context manager.
Using a class that doesn't define the __enter__ method as a context manager.
Forgetting to instantiate a class with parentheses () when using it as a context manager.
Reassigning the built-in open() function to a different object.
Using the read() method directly in your with statement.

# Using Context managers correctly

A context manager is an object that defines a context when using the with statement.

You will most commonly use context managers to close files automatically using the with statement.

main.py

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with open('example.txt', 'r', encoding='utf-8') as file:
    # 👇️ ['bobby\n', 'hadz\n', 'com\n']
    print(file.readlines())

The code sample above assumes that you have an example.txt file in the same directory as your Python script.

example.txt

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bobby
hadz
com

using context manager to open file

The advantage of using a context manager (the with statement) is that the file gets automatically closed once you exit the with object.

If you call the open() function directly, you have to close the file yourself.

main.py

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file = open('example.txt', 'r', encoding='utf-8')

# 👇️ ['bobby\n', 'hadz\n', 'com\n']
print(file.readlines())

file.close()

using open function directly

Using the open() function directly is not recommended because if an unhandled exception occurs, you might not get to close the file.

This would cause a memory leak in your application.

# Using a string as a context manager

A common cause of the error is trying to use a string as a context manager.

Here is an example.

main.py

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file_name = 'example.txt'

# ⛔️ AttributeError: __enter__
with file_name as file_obj:
    print(file_obj.readlines())

using string as context manager

Notice that the file_name variable stores a string.

The with statement is used with an object, not a string.

To solve the error, pass the file_name to the open() function.

main.py

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file_name = 'example.txt'

with open(file_name, 'r', encoding='utf-8') as file_obj:

    # 👇️ ['bobby\n', 'hadz\n', 'com\n']
    print(file_obj.readlines())

We used the open() function to open and read the file which resolved the issue.

# Using a context manager without defining `enter`

Here is an example of how the error occurs.

main.py

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class EmployeeContextManager:
    def __init__(self):
        print('Initializing the context manager')


# ⛔️ AttributeError: __enter__
with EmployeeContextManager() as emp_obj:
    print('Employee object:', emp_obj)

attributeerror enter

shell

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Initializing the context manager
Traceback (most recent call last):
  File "/home/borislav/Desktop/bobbyhadz_python/main.py", line 6, in <module>
    with EmployeeContextManager() as emp_obj:
AttributeError: __enter__

The issue in the code sample is that we are using a class as a context manager (with the with statement), but we haven't defined the __enter__ and __exit__ methods.

If you got the error when using a Context Manager class from a third-party library, you probably have instantiated the class incorrectly in the with statement.

If you get the error when creating a Context Manager class, you have to define the __enter__ and __exit__ methods.

main.py

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class EmployeeContextManager:
    def __init__(self):
        print('Initializing the context manager')

    def __enter__(self):
        print('__enter__ method ran')
        return self

    def __exit__(self, exc_type, exc_value, exc_traceback):
        print('__exit__ method ran')


with EmployeeContextManager() as emp_obj:
    print('Employee object:', emp_obj)

The code sample produces the following output.

shell

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Initializing the context manager
__enter__ method ran
Employee object: <__main__.EmployeeContextManager object at 0x7f8b6b91c2d0>
__exit__ method ran

defining enter and exit methods

The __enter__ method enters the runtime context related to the object.

Whatever you return from the __enter__ method gets assigned to the variable in the with statement.

The __exit__ method exits the runtime context related to the object.

The parameters of the method describe the exception that caused the context to be exited.

If the context is exited without an exception, the three arguments of the method are set to None.

Both the __enter__ and __exit__ methods have to be defined.

If you only define the __enter__ method, you would get an "AttributeError: __exit__" exception.

# Misspelling the names of the `enter` or `exit` methods

Make sure you haven't misspelled the names of the __enter__ and __exit__ methods.

The Python interpreter looks for methods with matching names, so if it doesn't find them, the error is raised.

# Forgetting to instantiate the class with parentheses

Another common cause of the error is forgetting to instantiate the class with parentheses when using it as a context manager.

Here is an example.

main.py

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class EmployeeContextManager:
    def __init__(self):
        print('Initializing the context manager')

    def __enter__(self):
        print('__enter__ method ran')
        return self

    def __exit__(self, exc_type, exc_value, exc_traceback):
        print('__exit__ method ran')


# ⛔️ AttributeError: __enter__
with EmployeeContextManager as emp_obj:
    print('Employee object:', emp_obj)

missing parentheses when calling context manager

To solve the error, invoke the context manager with parentheses (), as you would invoke a function or instantiate a class.

main.py

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class EmployeeContextManager:
    def __init__(self):
        print('Initializing the context manager')

    def __enter__(self):
        print('__enter__ method ran')
        return self

    def __exit__(self, exc_type, exc_value, exc_traceback):
        print('__exit__ method ran')


#     👇️ Added parentheses ()
with EmployeeContextManager() as emp_obj:
    print('Employee object:', emp_obj)

When the parentheses are missing, we are trying to use the class as a context manager instead of an instance of the class.

# Reassigning the open() function in your code

The error also commonly occurs when you reassign the open() function, setting it to something else.

main.py

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def example_func(file_name, mode):
    print(file_name, mode)


# 👇️ Reassigning open() function
open = example_func

file_name = 'example.txt'

# ⛔️ AttributeError: __enter__
with open(file_name, 'r') as file_obj:
    print(file_obj.read())

We reassigned the open() function and set it to a different function.

When we try to open the file, we get the error because example_func is not a context manager.

To solve the error, you have to remove the reassignment and use the native, built-in open() function.

main.py

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file_name = 'example.txt'

# ✅ Works as expected
with open(file_name, 'r', encoding='utf-8') as file_obj:
    print(file_obj.read())

Common sources of reassigning a function by mistake are:

Directly assignment, e.g. open = another_function.
Importing an open function from a different module (shadowing built-in open function).
Defining a function called open yourself.

If you defined a function called open, you have to rename it because it shadows the built-in open function.

You can print the open function in your code to make sure it points to the correct value.

main.py

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# 👇️ <built-in function open>
print(open)

# Using the `read()` method incorrectly

Here is another example of how the error occurs.

main.py

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file_name = 'example.txt'

# ⛔️ AttributeError: __enter__
with open(file_name, 'r', encoding='utf-8').read() as file_obj:

    print(file_obj)

Notice that we used the read() method directly after the call to the open() function.

This causes the error because the read() method returns a string and we are trying to use a string as a context manager.

To solve the error, call the read() method inside the with statement, after the file has been opened.

main.py

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file_name = 'example.txt'


with open(file_name, 'r', encoding='utf-8') as file_obj:
    # ✅ Calling read() inside with statement

    # bobby
    # hadz
    # com
    print(file_obj.read())

Once you move the call to the read() method inside the with block, the error is resolved.

# Using function-based context managers

main.py

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from contextlib import contextmanager


@contextmanager
def employee_context_manager():
    print('Entering the context manager')

    yield 'context manager return value'

    print('Exiting the context manager')


with employee_context_manager() as emp_context:
    print(emp_context)

Running the code sample produces the following output.

output

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Entering the context manager
context manager return value
Exiting the context manager

using function based context manager

Notice that we didn't have to define __enter__ and __exit__ methods.

The code that is placed before the yield statement is the initialization code (the __enter__ method).

The yield statement is used to return a value that gets assigned to the variable in the with statement.

The emp_context variable in the example is set to the string 'context manager return value'.

The code after the yield statement is the teardown section (the __exit__ method).

Function-based context managers require you to define less boilerplate code than class-based ones.

# Conclusion

To solve the Python "AttributeError: __enter__" exception, make sure:

You haven't used a string as a context manager.
You have defined the __enter__ and __exit__ methods in your class context managers.
You haven't forgotten to instantiate the class with parentheses () in the with statement.
You haven't reassigned the built-in open function.
You haven't used the read() method directly in your with statement.

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AttributeError: __enter__ exception in Python [Solved]