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In Python, arguments are passed by object reference (also called call by sharing). This means the function receives a reference to the same object, but reassigning the parameter inside the function won’t affect the caller’s variable — only mutating a mutable object will.
Example: Mutable vs Immutable
def modify_list(lst):lst.append(4) # in-place modificationdef reassign_list(lst):lst = [99] # rebinds local name, no effect outsidenums = [1, 2, 3]modify_list(nums)print(nums) # [1, 2, 3, 4] - changed outside tooreassign_list(nums)print(nums) # [1, 2, 3, 4] - unchangedCopied!✕CopyHere, modify_list mutates the original list (mutable), so changes persist. reassign_list points lst to a new list locally — the caller’s reference is unaffected.
Working with Immutable Types
Immutable objects (like int, str, tuple) can’t be changed in place. To “simulate” pass-by-reference for them:
Return and reassign:
def increment(x):return x + 1n = 5n = increment(n)print(n) # 6Copied!âś•CopyWrap in a mutable container:
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Code sample
def use_a_wrapper_to_simulate_pass_by_reference(stuff_to_change):new_string = something_to_do_with_the_old_string(stuff_to_change[0])stuff_to_change[0] = new_stringwrapper = [my_string]use_a_wrapper_to_simulate_pass_by_reference(wrapper)...Is Python call by reference or call by value
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