PYTHON Reference Guide

Revision Time: 4 mins

Dictionary Methods Reference

Safe lookups, key extractions, and value assignments on dicts.

get

Return: T

Get value for key safely, returning a default if not found.

Used In: Safe config loading, API response parsing.

Syntax signature:dict.get(key, default=None)

Code snippet:

python

d = {'id': 1}
print(d.get('name', 'Unknown'))

Expected Output:'Unknown'

Time Complexity: O(1)

Common Mistakes: Confusing dict.get('key') returning None with a key that is present but holds None.

Related Methods: setdefault(), keys()

Comparison:

dict[key] vs dict.get(key): dict[key] raises KeyError if key is missing; dict.get(key) returns a default value safely.

Remember: Avoids raising KeyError on missing keys.

keys

Return: dict_keys

Return a dynamic view object of all dictionary keys.

Used In: Membership iterations, checking keys schemas.

Syntax signature:dict.keys()

Code snippet:

python

d = {'a': 1, 'b': 2}
print(list(d.keys()))

Expected Output:['a', 'b']

Time Complexity: O(1)

Related Methods: values(), items()

Remember: Keys view dynamically updates if the dictionary changes.

values

Return: dict_values

Return a dynamic view object of all dictionary values.

Used In: Summing counts, checking status lists.

Syntax signature:dict.values()

Code snippet:

python

d = {'a': 1, 'b': 2}
print(list(d.values()))

Expected Output:[1, 2]

Time Complexity: O(1)

Related Methods: keys(), items()

Remember: Allows fast checking of dictionary values contents.

items

Return: dict_items

Return a dynamic view object of (key, value) tuple pairs.

Used In: Looping dictionary properties in ETL data normalization.

Syntax signature:dict.items()

Code snippet:

python

d = {'a': 1}
for k, v in d.items():
    print(k, v)

Expected Output:a 1

Time Complexity: O(1)

Related Methods: keys(), values()

Remember: Allows unpacking key and value directly in loop definitions.

update

Return: None

Update dictionary with keys/values from another dict or key-value pairs.

Used In: Merging config dictionaries, combining metadata feeds.

Syntax signature:dict.update([other])

Code snippet:

python

d = {'a': 1}
d.update({'b': 2})
print(d)

Expected Output:{'a': 1, 'b': 2}

Time Complexity: O(K) where K is items count in other

Related Methods: union operator (|)

Remember: Updates existing keys and appends new keys in-place.

pop

Return: T

Remove specified key and return its corresponding value.

Used In: Extracting and deleting fields from JSON payloads.

Syntax signature:dict.pop(key[, default])

Code snippet:

python

d = {'id': 1}
v = d.pop('id')
print(v, d)

Expected Output:1 {}

Time Complexity: O(1)

Related Methods: popitem()

Remember: Raises KeyError if key is missing and no default parameter is specified.

popitem

Return: Tuple[K, V]

Remove and return the last inserted (key, value) pair.

Used In: Destructive buffer consumption.

Syntax signature:dict.popitem()

Code snippet:

python

d = {'a': 1, 'b': 2}
k, v = d.popitem()
print(k, v)

Expected Output:'b' 2

Time Complexity: O(1)

Related Methods: pop()

Remember: Raises KeyError if the dictionary is empty. Follows LIFO (last-in, first-out) order.

setdefault

Return: T

Get key value if present; else set key to default and return default.

Used In: Grouping records, indexing items.

Syntax signature:dict.setdefault(key, default=None)

Code snippet:

python

d = {}
d.setdefault('errors', []).append('OOM')
print(d)

Expected Output:{'errors': ['OOM']}

Time Complexity: O(1)

Common Mistakes: Calling dict.setdefault(key, []) evaluates the default list [] on every call, although it is only assigned if the key is missing.

Related Methods: collections.defaultdict

Comparison:

dict.setdefault() vs collections.defaultdict: setdefault() is built-in but evaluates default parameters immediately; defaultdict lazily calls its factory function only when key is missing.

Remember: Useful for grouping elements or building dependency lists.

clear

Return: None

Remove all items from the dictionary.

Used In: Clearing global lookup tables between batches.

Syntax signature:dict.clear()

Code snippet:

python

d = {'a': 1}
d.clear()
print(d)

Expected Output:{}

Time Complexity: O(1)

Remember: Clears the dict in-place, keeping references valid.

copy

Return: Dict[K, V]

Return a shallow copy of the dictionary.

Used In: Isolating parameter configurations.

Syntax signature:dict.copy()

Code snippet:

python

d = {'a': [1]}
dup = d.copy()
dup['a'].append(2)
print(d)

Expected Output:{'a': [1, 2]}

Time Complexity: O(N)

Related Methods: copy.deepcopy()

Remember: Mutating lists or dicts nested inside a shallow copy edits the original.

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