Immutable vs Mutable Objects in Python
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What defines an object as immutable in Python?

An immutable object cannot be changed in place after it is created; any operation that seems to modify it actually creates a new object.

Provide an example of an immutable object and explain what happens during concatenation.

An example is a string like 'hello'. When concatenated with ' world', a new string 'hello world' is created instead of modifying the original string.

How do mutable objects differ from immutable objects in terms of memory usage during modification?

Mutable objects do not use new memory on modification, while immutable objects create a new copy each time they are modified.

List two examples of mutable objects in Python.

<p>Lists and dictionaries are two examples of mutable objects in Python.</p> Signup and view all the answers

Explain a benefit of using immutable objects in multi-threaded programming.

<p>Immutable objects are safer in multi-threading as they prevent accidental modifications, ensuring data consistency.</p> Signup and view all the answers

What happens to a list when the append method is called?

<p>When the append method is called, the list is modified in place by adding the new element to its existing structure.</p> Signup and view all the answers

What are the implications of using immutable objects concerning performance?

<p>Using immutable objects can be inefficient when frequent changes are made, as each modification creates a new copy.</p> Signup and view all the answers

Explain what it means for a tuple to be immutable and provide its significance.

<p>A tuple is immutable, meaning its elements cannot be changed after it is created, which ensures data integrity.</p> Signup and view all the answers

What is the purpose of the eq method in a custom object used as a dictionary key?

<p><strong>eq</strong> defines equality between objects to handle hash collisions, allowing different objects to be considered equal if their values match.</p> Signup and view all the answers

Why can't mutable objects, like lists, be used as keys in a dictionary?

<p>Mutable objects cannot be used as keys because their hash values can change if they are modified, leading to incorrect dictionary lookups.</p> Signup and view all the answers

Explain the significance of hash stability concerning keys in a dictionary.

<p>Hash stability ensures that the hash value of a key remains constant throughout its lifespan, allowing it to be reliably used for lookups in the dictionary.</p> Signup and view all the answers

What happens if you try to hash a mutable object in Python, and what is the resulting error?

<p>Attempting to hash a mutable object, like a list, will raise a TypeError, specifically 'unhashable type: list'.</p> Signup and view all the answers

Provide an example of a custom class that can be hashed and explain its methods.

<p>A custom class can implement both <strong>hash</strong> and <strong>eq</strong> methods; for example, 'class CustomKey: def <strong>hash</strong>(self): return hash(self.value) def <strong>eq</strong>(self, other): return isinstance(other, CustomKey) and self.value == other.value.'</p> Signup and view all the answers

What type of objects can be used as dictionary keys in Python?

<p>Only immutable objects can be used as dictionary keys.</p> Signup and view all the answers

Why can't mutable objects like lists be used as dictionary keys?

<p>Mutable objects cannot be used because their content can change, making them unreliable for key lookups.</p> Signup and view all the answers

What error is raised when you attempt to use a mutable object like a list as a dictionary key?

<p>A TypeError is raised, stating 'unhashable type: list'.</p> Signup and view all the answers

Which method is used in Python to determine an object's hash value for dictionary keys?

<p>The <code>__hash__</code> method is used to compute an object's hash value.</p> Signup and view all the answers

Explain the role of the __eq__ method in the context of dictionary keys?

<p>The <code>__eq__</code> method is used to check for equality between keys to resolve collisions.</p> Signup and view all the answers

Provide two examples of valid dictionary keys.

<p>Examples of valid keys include an integer like 1 and a tuple like (2, 3).</p> Signup and view all the answers

What happens if a dictionary uses a mutable object as a key?

<p>Using a mutable object as a key will raise a TypeError immediately.</p> Signup and view all the answers

Can a frozenset be used as a dictionary key? Why or why not?

<p>Yes, a frozenset can be used as a dictionary key because it is an immutable type.</p> Signup and view all the answers

Which immutable collection type can be used as a dictionary key in Python?

<p>frozenset</p> Signup and view all the answers

What does LEGB stand for in the context of Python variable scope?

<p>Local, Enclosed, Global, Built-in</p> Signup and view all the answers

What will happen if you attempt to access a local variable outside of its function?

<p>NameError: name 'x' is not defined</p> Signup and view all the answers

How does Python determine which variable to use when multiple scopes are involved?

<p>Python searches from the local scope to the built-in scope in order.</p> Signup and view all the answers

What is the effect of declaring a variable as global if it is modified inside a function?

<p>The global variable can be modified.</p> Signup and view all the answers

Can a list be used as a key in a Python dictionary?

<p>No, lists are not hashable.</p> Signup and view all the answers

What will be printed if 'inner' is called within 'outer' and x is found in the enclosing scope?

<p>x will output 'enclosing'.</p> Signup and view all the answers

What happens if a local variable inside a nested function has the same name as a global variable?

<p>The local variable shadows the global variable.</p> Signup and view all the answers

What is the purpose of using the nonlocal keyword in Python?

<p>The <code>nonlocal</code> keyword allows a nested function to modify a variable from the enclosing scope.</p> Signup and view all the answers

In the provided example, what would the output be if nonlocal was not used?

<p>If <code>nonlocal</code> was not used, the output would be an error since <code>x</code> would be treated as a local variable within <code>inner</code>.</p> Signup and view all the answers

When should you use global instead of nonlocal?

<p><code>global</code> should be used when you need to modify a variable at the module level, outside any function.</p> Signup and view all the answers

Can nonlocal be used to change a variable that is in the global scope?

<p>No, <code>nonlocal</code> cannot be used to change variables in the global scope; it only works for variables in the nearest enclosing scope.</p> Signup and view all the answers

What happens if you declare a variable as nonlocal but it doesn't exist in any enclosing scope?

<p>An error will occur because <code>nonlocal</code> requires the variable to already exist in an enclosing scope to modify it.</p> Signup and view all the answers

What happens when a global variable is modified within a function without using the 'global' keyword?

<p>An UnboundLocalError occurs because Python treats the variable as local and references it before assignment.</p> Signup and view all the answers

How can you modify a global variable inside a function?

<p>By using the 'global' keyword before the variable name within the function.</p> Signup and view all the answers

What is the significance of the Enclosed scope in nested functions?

<p>Enclosed scope allows an inner function to access variables from its enclosing function.</p> Signup and view all the answers

Why does using the 'global' keyword not work for variables in the Enclosed scope?

<p>The 'global' keyword searches for the variable in the global namespace, not in the enclosing function scope.</p> Signup and view all the answers

What will the code def change(): x = x + 1 produce if 'x' is not declared as global?

<p>It will raise an UnboundLocalError.</p> Signup and view all the answers

Provide an example of accessing a variable in the Enclosed scope.

<p>In the function 'inner()', you can print 'x' defined in 'outer()' without any issue.</p> Signup and view all the answers

What is the output of the function call change() when 'x' is 10 and declared as global?

<p>The output will be 11.</p> Signup and view all the answers

Why is the usage of 'global' considered poor practice in larger programs?

<p>It complicates debugging and makes the code harder to maintain.</p> Signup and view all the answers

What value does the inner function print in the example 'def outer()' with the enclosed variable 'x'?

<p>'It prints the value 'enclosed'.</p> Signup and view all the answers

What will the output of outer() be when it contains a nested function inner() that accesses 'x'?

<p>'It will output 'enclosed'.</p> Signup and view all the answers

Study Notes

Immutable vs Mutable Objects

  • Immutable Objects: Cannot be changed after creation. Operations appear to modify but actually create new objects.
  • Examples of Immutable Objects:
    • Numbers (int, float, complex)
    • Strings (str)
    • Tuples (tuple)
    • Frozen sets (frozenset)
  • Immutability Example: Modifying a string creates a new string.
    • x = "hello"
    • x = x + " world" (creates a new string).
  • Mutable Objects: Can be changed in place after creation. Content can be modified without creating a new object.
  • Examples of Mutable Objects:
    • Lists (list)
    • Dictionaries (dict)
    • Sets (set)

Key Differences

Feature Mutable Objects Immutable Objects
Changeable in place Yes No
Memory Use on Modification No Yes
Examples list, dict, set int, str, tuple

Implications of Mutability

  • Performance: Immutable objects create new copies (inefficient for frequent changes).
  • Multi-threading: Immutable objects prevent accidental modifications, improving safety.
  • Dictionary Keys & Set Elements: Only immutable objects can be used as keys or elements.

Dictionary Key Type Rules

  • Only immutable objects can be dictionary keys (e.g., int, str, tuple, frozenset).
  • Mutable objects (e.g., list, dict, set) cannot be used as keys because their content changes, making them unreliable.

Invalid Keys (Cause TypeError)

  • List [1, 2] and dictionary {3:4} cannot be used as dictionary keys.

Hash Method

  • Returns a unique integer representing an object's hash value.
  • Required for objects to be used as dictionary keys or set elements.
  • Immutable objects have stable hash values.

eq Method

  • Defines equality between objects. Important for resolving hash collisions.

Why Keys Must Be Immutable

  • Hash Stability: If an object changes, its hash changes, making it impossible for dictionaries and sets to locate the object. Changes to a dictionary key would disrupt the hash table lookup.

LEGB Rule (Scope Resolution)

  • Local (L): Variables defined inside a function.
  • Enclosing Function Locals (E): Variables in enclosing functions (nested functions).
  • Global (G): Variables defined at the top level of a module or script.
  • Built-in (B): Predefined functions and variables in Python.
  • Python searches for variables in this order, stopping at the first match.
  • Modifying global variables inside function, you must explicitly use the global keyword.

Global Scope

  • Global variables are defined at the top level of a module or script.
  • They are accessible throughout the module or script.
  • Unlike other scopes, global explicitly declares that a variable belongs to global scope.

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Description

This quiz covers the key differences between immutable and mutable objects in Python. Learn about various examples of each type, their implications on performance, and how they behave in memory. Test your understanding of these concepts and their impact on programming in Python.

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