Вивчайте Copy vs Reference – copy and deepcopy

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Assigning an object to a new variable does not copy it – both variables point to the same object. Modifying the object through one variable affects the other. Understanding when Python copies and when it references is essential for avoiding unexpected mutations and unnecessary memory duplication.

Assignment Creates References, Not Copies


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import sys

# Assignment – both variables point to the same object
original_budget = {"q1": 50000, "q2": 75000, "q3": 90000}
budget_ref = original_budget  # No copy – same object

budget_ref["q1"] = 99999  # Modifying through budget_ref
print(original_budget)    # {'q1': 99999, 'q2': 75000, 'q3': 90000} – original changed

print(original_budget is budget_ref)  # True – same object
print(sys.getrefcount(original_budget))  # At least 3

Shallow Copy

A shallow copy creates a new container object but does not copy the objects inside it – nested objects are still shared:


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import copy

# Shallow copy – new dict, but nested lists are shared
portfolio = {
    "equities": [150000, 200000, 175000],
    "bonds": [80000, 95000],
}

portfolio_shallow = copy.copy(portfolio)

portfolio_shallow["equities"].append(250000)  # Modifies shared nested list

print(portfolio["equities"])          # [150000, 200000, 175000, 250000] – modified!
print(portfolio_shallow["equities"])  # [150000, 200000, 175000, 250000] – same object

print(portfolio is portfolio_shallow)           # False – different dicts
print(portfolio["equities"] is portfolio_shallow["equities"])  # True – shared list

For flat structures with no nested mutable objects, shallow copy is sufficient and cheaper.

Deep Copy

copy.deepcopy() recursively copies all nested objects. The result is completely independent:


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import copy

# Deep copy – fully independent copy of nested structure
portfolio = {
    "equities": [150000, 200000, 175000],
    "bonds": [80000, 95000],
}

portfolio_deep = copy.deepcopy(portfolio)

portfolio_deep["equities"].append(250000)  # Only modifies the copy

print(portfolio["equities"])       # [150000, 200000, 175000] – unchanged
print(portfolio_deep["equities"])  # [150000, 200000, 175000, 250000]

Memory Cost of Deep Copy

Deep copy allocates memory for every nested object. For large structures, this can be expensive:


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import copy
import sys
import tracemalloc

# Measuring memory cost of deep copy vs shallow copy
large_dataset = {
    f"region_{region_id}": list(range(1000))
    for region_id in range(100)
}

tracemalloc.start()

shallow = copy.copy(large_dataset)
_, shallow_peak = tracemalloc.get_traced_memory()
tracemalloc.clear_traces()

deep = copy.deepcopy(large_dataset)
_, deep_peak = tracemalloc.get_traced_memory()

tracemalloc.stop()

print(f"Shallow copy peak: {shallow_peak / 1024:.1f} KB")
print(f"Deep copy peak:    {deep_peak / 1024:.1f} KB")

Choosing the Right Approach

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