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  1. LPU MCA Semester 1
  2. CAB106
  3. Unit 4

CAB106 · Unit 4: Abstraction, modules and packages

Modules, import statements and packages

11 practice questions

Practise this topicPractise this topic

Key points

  • A module is a .py file; a package is a folder with __init__.py
  • ModuleNotFoundError for a missing module, ImportError for a missing name
  • Imports are cached in sys.modules
  • The __main__ guard runs code only on direct execution

A module is a single .py file whose names can be reused with import. A package is a folder of modules, marked by an __init__.py file. Common forms are import math (use math.floor), from math import sqrt (use sqrt directly), import math as m and from math import sqrt as r. A star import such as from math import * copies every public name and can overwrite your own names, so it is discouraged.

SituationResult
Module not foundModuleNotFoundError
Name missing from an existing moduleImportError
Same module imported twiceCached in sys.modules, one object

Inside a package a sibling module is imported with a relative import such as from . import b; one dot means the current package and two dots mean the parent. Every module has a __name__. When a file is run directly __name__ is "__main__", so the guard if __name__ == "__main__": runs code only for direct execution, not on import. importlib.import_module("name") imports by string, and os.path.basename is an example of a function in a submodule.

Exam trap: importing a module twice does not run it twice. Python caches it in sys.modules.

Try two questions

Question 1 · Tier 2, Understand

Which file marks a folder as a regular Python package?

  1. __main__.py
  2. __init__.py
  3. setup.py
  4. package.py
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B. A folder containing __init__.py is treated as a regular package.

Why not A: __main__.py makes a package runnable but does not mark it as a package.

Why not C: setup.py is used for installing, not for marking a package.

Why not D: Python has no special file called package.py.

Question 2 · Tier 4, Analyse

Inside a package, module a wants to import module b from the same package. Which relative import is correct?

  1. import .b
  2. from .. import b
  3. from . import b
  4. from b import .
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C. One dot means the current package, so from . import b imports its sibling module.

Why not A: The dot form cannot follow a plain import statement.

Why not B: Two dots refer to the parent package, not the current one.

Why not D: That is not valid import syntax.