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Quiz: Standard Library and JSON

Test your understanding of Python modules, JSON data, the sys and os modules, and the collections module with these questions. Click "Show Answer" to reveal the correct answer and explanation.

1. What is a Python module?

  1. A built-in function like print() or len()
  2. A .py file containing functions, variables, and classes that can be imported and reused
  3. A special comment that tells Python to load extra features
  4. A folder containing multiple Python programs
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The correct answer is B. A module is simply a .py file. Any Python file can be a module — you write functions and variables in it, then use import module_name in another file to access them. Python also ships with many pre-built modules (the standard library) like math, random, os, sys, and json.

Concept Tested: Custom Modules

2. What does the if __name__ == "__main__": guard do?

  1. It makes the program run faster by skipping imports
  2. It checks whether the module has been imported before
  3. It ensures the indented block runs only when the file is executed directly, not when it is imported
  4. It defines the entry point for a web server
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The correct answer is C. When Python runs a file directly, __name__ equals "__main__". When a file is imported as a module, __name__ equals the module's filename. The guard if __name__ == "__main__": is used to put test code or script logic that should only run when you execute the file directly — not when another program imports it.

Concept Tested: name Guard

3. What does sys.argv contain?

  1. The list of all modules currently imported into the program
  2. The command-line arguments passed to the Python script when it was run
  3. A list of all functions defined in the program
  4. The Python interpreter's version and platform information
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The correct answer is B. sys.argv is a list of strings representing the command-line arguments. sys.argv[0] is always the script name itself. If you run python my_script.py hello 42, then sys.argv equals ["my_script.py", "hello", "42"]. This allows programs to accept configuration from the command line without using input().

Concept Tested: sys Module

4. What does os.path.exists("data.txt") return?

  1. The full path to data.txt on your computer
  2. True if the file or folder exists, False if it does not
  3. The file size in bytes
  4. The date and time when the file was last modified
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The correct answer is B. os.path.exists(path) checks whether a file or directory exists at the given path and returns True or False. This is useful before trying to open a file — you can check first to avoid a FileNotFoundError. The os.path sub-module has many other useful functions like os.path.join() and os.path.basename().

Concept Tested: os Module

5. What is JSON?

  1. A Python-specific binary format for saving program data
  2. A text format for storing structured data that looks like Python dictionaries and lists
  3. A type of encrypted file used for storing passwords securely
  4. A database language similar to SQL
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The correct answer is B. JSON (JavaScript Object Notation) is a lightweight text format for structured data. It uses curly braces {} for objects (like Python dictionaries), square brackets [] for arrays (like Python lists), and true/false/null for booleans and None. JSON is the most common format for web APIs and configuration files, and Python's json module makes it easy to read and write.

Concept Tested: JSON Format

6. What does json.loads('{"x": 1}') return?

  1. The string '{"x": 1}' unchanged
  2. A Python dictionary {"x": 1}
  3. A Python list [{"x": 1}]
  4. An error because JSON requires double quotes around keys
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The correct answer is B. json.loads(text) parses a JSON string and returns the equivalent Python object. {"x": 1} in JSON becomes a Python dict. The function name "loads" stands for "load string." JSON uses double quotes for strings, which is valid Python when the string is inside single quotes. The result is a native Python dictionary you can use like any other.

Concept Tested: json.loads()

7. What does json.dumps({"name": "Alice", "age": 14}) do?

  1. Saves the dictionary directly to a file named Alice.json
  2. Prints the dictionary to the console in JSON format
  3. Converts the Python dictionary to a JSON-formatted string
  4. Loads the dictionary from a JSON file
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The correct answer is C. json.dumps(obj) converts a Python object (dict, list, number, string, bool, or None) into a JSON-formatted string. The name "dumps" stands for "dump string." To save JSON to a file, use json.dump(obj, file_object) instead. json.dumps() is useful when you need a JSON string in memory, for example to send over a network.

Concept Tested: json.dumps()

8. What Python type does JSON true map to?

  1. The string "true"
  2. The integer 1
  3. The Python boolean True
  4. The Python value None
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The correct answer is C. JSON uses lowercase true and false for booleans; Python uses capitalized True and False. When json.loads() encounters true in JSON, it converts it to Python's True. Similarly, JSON false becomes Python False, and JSON null becomes Python None.

Concept Tested: JSON Type Mapping

9. What does os.path.join("folder", "file.txt") produce?

  1. "folder file.txt" (with a space)
  2. "folder/file.txt" on Mac/Linux or "folder\file.txt" on Windows, using the correct separator for the operating system
  3. Always "folder/file.txt" regardless of operating system
  4. An error because you cannot join paths without a leading slash
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The correct answer is B. os.path.join() builds a file path using the correct separator for the current operating system — forward slash / on Mac and Linux, backslash \ on Windows. Never manually concatenate path strings with "/" because your code will break on Windows. Always use os.path.join() for portable, cross-platform path construction.

Concept Tested: os.path

10. What is the difference between json.loads()/json.dumps() and json.load()/json.dump()?

  1. loads/dumps are faster; load/dump produce better formatted output
  2. loads/dumps work with strings; load/dump work with file objects
  3. loads reads one line; load reads the entire file
  4. They are identical — the extra s is just an alias
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The correct answer is B. json.loads(text) parses from a string; json.dumps(obj) serializes to a string. json.load(file) reads JSON directly from an open file object; json.dump(obj, file) writes JSON directly to an open file object. Use the s versions (loads/dumps) for in-memory strings; use the no-s versions (load/dump) when working with files.

Concept Tested: json Module Functions