Python exception handling: ImportError and ModuleNotFoundError (2023)

Make your way through our detailedPython exception handlingSeries we've arrived atImportError, along with its only child subclass ofModuleNotFoundError. ÖImportErrorwill be canceled if aimportThe statement has problems importing the specified module successfully. Usually this problem occurs due to an invalid or incorrect path generating anModuleNotFoundErrorin Python 3.6 and newer.

In this article, we examine theImportErroreModuleNotFoundErrorwith a bit more detail starting with where they sit overallPython exception class hierarchy. We'll also look at some simple code examples that illustrate the differences inimportStatement errors in newer (3.6) and older (2.7) versions of Python, so let's get started!

the technical summary

All Python exceptions inherit fromBaseExceptionclass or from a class inherited from it. The complete exception hierarchy for this error is:

• BaseException
  • exception
    • ImportError
      • ModuleNotFoundError

Complete code sample

Below is the complete code example that we will use in this article. It can be copied and pasted if you want to play around with the code and see how it all works.

# outer_import_2.7.py
Import system
import gw_utility.Book
def principal():
experiment:
print(sys.version)
except ImportError as error:
# Expected output from ImportErrors.
print(error.__class__.__name__ + ": " + error.message)
except exception as exception:
# Throw unexpected exceptions.
print(Exception, false)
print(Exception.__Class__.__Name__ + ": " + Exception.Message)
se __name__ == "__main__":
a director()

# inner_import_2.7.py
Import system
def principal():
experiment:
print(sys.version)
import gw_utility.Book
except ImportError as error:
# Expected output from ImportErrors.
print(error.__class__.__name__ + ": " + error.message)
except exception as exception:
# Throw unexpected exceptions.
print(Exception, false)
print(Exception.__Class__.__Name__ + ": " + Exception.Message)
se __name__ == "__main__":
a director()

# outer_import_3.6.py
Import system
import gw_utility.Book
en gw_utility.logging import Logging
def principal():
experiment:
logging.log (sys.version)
except ImportError as error:
# Expected output from ImportErrors.
Logging.log_exception (Error)
# Include the name and path attributes in the output.
Logging.log(f'error.name: {error.name}')
Logging.log(f'error.path: {error.path}')
except exception as exception:
# Throw unexpected exceptions.
Logging.log_exception(Exception, False)
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se __name__ == "__main__":
a director()

# inner_import_3.6.py
Import system
en gw_utility.logging import Logging
def principal():
experiment:
logging.log (sys.version)
import gw_utility.Book
except ImportError as error:
# Expected output from ImportErrors.
Logging.log_exception (Error)
# Include the name and path attributes in the output.
Logging.log(f'error.name: {error.name}')
Logging.log(f'error.path: {error.path}')
except exception as exception:
# Throw unexpected exceptions.
Logging.log_exception(Exception, False)
se __name__ == "__main__":
a director()

# log.py
Import math
Import system
Import tracking
Class registration:
separador_character_default = '-'
separador_length_default = 40
@Class method
def __output(cls, *args, sep: str = ' ', end: str = '\n', file=None):
"""Prints the value(s) passed to the console.
:param args: Values ​​to be output.
:param sep: String inserted between values, a space by default.
:param end: String appended after the last value, default is a newline.
:param file: A file-like object (stream); the default is the current sys.stdout.
:back: None
"""
print(*args, set=set, end=end, file=file)
@Class method
def line_separator(cls, Wert: str = None, Länge: int = separator_length_default,
char: str = separador_character_default):
"""Prints a line separator with the entered text centered in the middle.
:param value: Inserted text to be centered.
:param length: Total length of the delimiter.
:param char: delimiter.
"""
output = value
# If no value is passed, the delimiter for the output length.
if value == None oder len(value) == 0:
output = f'{characters * length}'
elif len(value) < length:
# Update the length based on the length of the insert minus a space for the margin.
length -= length(value) + 2
# Halve the length and bottom on the left.
left = math.floor(Länge / 2)
right = left
# If the number is odd, add the discarded remainder on the right.
if length % 2 != 0:
right += 1
# Circumferential insert with tabs.
output = f'{auto*left}{value}{character*right}'
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cls.__output(output)
@Class method
def log(cls, *args, sep: str = ' ', end: str = '\n', file=None):
"""Prints the value(s) passed to the console.
:param args: Values ​​to be output.
:param sep: String inserted between values, a space by default.
:param end: String appended after the last value, default is a newline.
:param file: A file-like object (stream); the default is the current sys.stdout.
"""
cls.__output(*args, set=set, end=end, file=file)
@Class method
def log_exception(cls, exception: BaseException, expected: bool = True):
"""Prints the BaseException passed to the console, including traceback.
:param exception: A BaseException to throw.
:param expected: Determines whether BaseException was expected.
"""
output = "[{}] {}: {}".format('EXPECTED' se esperado senão 'UNEXPECTED', type(exception).__name__, exception)
cls.__output(output)
exc_type, exc_value, exc_traceback = sys.exc_info()
traceback.print_tb(exc_traceback)

When should you use it?

the apparently simpleimportThe statement found in Python is actually quite complex when you look under the hood. At the most basic level, aimportThe statement is used to perform two tasks. It first tries to find the named module, loads and initializes it if necessary. It also automatically sets a name in the local namespace within the scope of the map.importExplanation. This local name can then be used to refer to the called module in scoped code below.

DuringimportInstruction is the most commonly used technique to gain access to the code of other modules, Python offers others as wellmethods and functionsthat make up the built-in import system. Developers can use certain features to have finer control over the import process.

Let's stick to the usual for our code examplesimportStatement most of us are used to. As mentioned in the introduction, the behavior in the event of a failureIt is importantdiffers depending on the version of Python. To illustrate, let's start with theouter_import_2.7.pyArchive:

# outer_import_2.7.py
Import system
import gw_utility.Book
def principal():
experiment:
print(sys.version)
except ImportError as error:
# Expected output from ImportErrors.
print(error.__class__.__name__ + ": " + error.message)
except exception as exception:
# Throw unexpected exceptions.
print(Exception, false)
print(Exception.__Class__.__Name__ + ": " + Exception.Message)
se __name__ == "__main__":
a director()
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ÖOutsidePrefix to filename indicates we are testing an "external" or global scopeimportexplanation ofgw_utility.Book. Running this code produces the following output:

Traceback (last most recent call):
Arquivo "C:\Users\Gabe\AppData\Local\JetBrains\Toolbox\apps\PyCharm-P\ch-0\172.3968.37\helpers\pydev\pydevd.py", linha 1599, in <module>
globals = debugger.run(setup['file'], None, None, is_module)
Arquivo "C:\Users\Gabe\AppData\Local\JetBrains\Toolbox\apps\PyCharm-P\ch-0\172.3968.37\helpers\pydev\pydevd.py", Zeile 1026, ausgeführt
pydev_imports.execfile(file, global, local) # Run the script
Arquivo "D:/work/Airbrake.io/Exceptions/Python/BaseException/Exception/ImportError/outer_import_2.7.py", Linha 3, in <module>
import gw_utility.Book
ImportError: No module named Book

The general problem here is that thegw_utility.BookModule does not exist. Actually the correct modulelowercase letters:gw_utility.book. Sinceimport-Declaration is at the top of the file, it exists outside of oursTry - exceptblock, then dieImportErrorwe get in the log is not captured - execution stopped completely when the error was thrown.

Alternatively, let's look at what happens when we move theimportstatement within aTry - exceptblock as seen ininner_import_2.7.py:

# inner_import_2.7.py
Import system
def principal():
experiment:
print(sys.version)
import gw_utility.Book
except ImportError as error:
# Expected output from ImportErrors.
print(error.__class__.__name__ + ": " + error.message)
except exception as exception:
# Throw unexpected exceptions.
print(Exception, false)
print(Exception.__Class__.__Name__ + ": " + Exception.Message)
se __name__ == "__main__":
a director()

Running this code - even with Python 2.7 - produces the same thingImportError, but we can capture them and further process the capturedImportError, if necessary:

2.7.14 (v2.7.14:84471935ed, 16. September 2017, 20:25:58) [MSC v.1500 64 Bit (AMD64)]
ImportError: No module named Book

ÖModuleNotFoundErrorwas added in Python 3.6 as a subclass ofImportErrorand an explicit reference to the same type of error that we saw in the 2.7 code above. For example, let's look at the exteriorimportExample in Python 3.6 withouter_import_3.6.py:

# outer_import_3.6.py
Import system
import gw_utility.Book
en gw_utility.logging import Logging
def principal():
experiment:
logging.log (sys.version)
except ImportError as error:
# Expected output from ImportErrors.
Logging.log_exception (Error)
# Include the name and path attributes in the output.
Logging.log(f'error.name: {error.name}')
Logging.log(f'error.path: {error.path}')
except exception as exception:
# Throw unexpected exceptions.
Logging.log_exception(Exception, False)
se __name__ == "__main__":
a director()

We do that here tooimportthe endTry - exceptblock, so running this code stops execution and produces the following output:

Traceback (last most recent call):
Arquivo "C:\Users\Gabe\AppData\Local\JetBrains\Toolbox\apps\PyCharm-P\ch-0\172.3968.37\helpers\pydev\pydevd.py", linha 1599, in <module>
globals = debugger.run(setup['file'], None, None, is_module)
Arquivo "C:\Users\Gabe\AppData\Local\JetBrains\Toolbox\apps\PyCharm-P\ch-0\172.3968.37\helpers\pydev\pydevd.py", Zeile 1026, ausgeführt
pydev_imports.execfile(file, global, local) # Run the script
Arquivo "C:\Users\Gabe\AppData\Local\JetBrains\Toolbox\apps\PyCharm-P\ch-0\172.3968.37\helpers\pydev\_pydev_imps\_pydev_execfile.py", linha 18, kein arquivo exec
exec(compile(content+"\n", file, 'exec'), glob, loc)
Arquivo "D:/work/Airbrake.io/Exceptions/Python/BaseException/Exception/ImportError/outer_import_3.6.py", Linha 3, in <module>
import gw_utility.Book
ModuleNotFoundError: No module named "gw_utility.Book"

The cause of this error is exactly the same as in version 2.7, but it is more specific in 3.6+ModuleNotFoundErrornow it is time. Also, we can detect these errors when theimportruns within aTry - exceptContext:

# inner_import_3.6.py
Import system
en gw_utility.logging import Logging
def principal():
experiment:
logging.log (sys.version)
import gw_utility.Book
except ImportError as error:
# Expected output from ImportErrors.
Logging.log_exception (Error)
# Include the name and path attributes in the output.
Logging.log(f'error.name: {error.name}')
Logging.log(f'error.path: {error.path}')
except exception as exception:
# Throw unexpected exceptions.
Logging.log_exception(Exception, False)
se __name__ == "__main__":
a director()

With this code we can generate the Python version and handle the error:

3.6.3 (v3.6.3:2c5fed8, 3.Oktober 2017, 18:11:49) [MSC v.1900 64 Bit (AMD64)]
[EXPECTED] ModuleNotFoundError: No module named "gw_utility.Book"
error.name: gw_utility.Book
error.path: None

We also exhibitNameeAwayattributes ofImportErrorobject added in Python 3.3 to specify the name of the module that was attempted to be imported, along with the path to the file that threw the exception, if applicable. In this case, our code is fairly simple, so unfortunately none of the attributes are particularly useful.

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