To loop through a Python dictionary with a position counter, use enumerate() with dict.items():
scores = {"Ada": 91, "Linus": 87, "Grace": 95}
for i, (name, score) in enumerate(scores.items()):
print(i, name, score)This prints each key–value pair with a zero-based traversal number. The number is not a dictionary key: use scores[name] to look up a value by its actual key.
Choose the loop for what you need
Dictionary iteration yields keys by default. Use the view that matches what the loop body needs:
| What you need | Loop | Values produced |
|---|---|---|
| Keys | for key in d: |
Each key |
| Values | for value in d.values(): |
Each value |
| Keys and values | for key, value in d.items(): |
Each key–value pair |
| Position and keys | for i, key in enumerate(d): |
A counter and each key |
| Position, keys, and values | for i, (key, value) in enumerate(d.items()): |
A counter and each key–value pair |
Python’s tutorial notes that the items() method retrieves a key and its corresponding value together. enumerate(iterable, start=0) pairs each item with a counter; pass start=1 when you want a one-based number for display, as described in the built-in functions documentation.
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Why the index is not a dictionary key
A dictionary is a mapping from keys to values, not a sequence addressed by position. In for i, (key, value) in enumerate(d.items()):, i counts entries as the loop visits them. It does not change the dictionary or make d[i] mean “the entry at position i.”
Dictionary subscription looks up a key. So d[i] works only if the dictionary actually contains the integer key i; otherwise it raises KeyError. To retrieve the value for the current pair, use d[key] or the value already unpacked from items(). If you want a fallback for a possibly absent key, use d.get(key, default).
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How dictionary order affects iteration
In Python 3.7 and later, insertion order is a language guarantee: keys are produced in the order they were added. Replacing the value of an existing key does not change its position; removing a key and inserting it again places it at the end. The Python data model reference documents this behavior and its version history. CPython 3.6 preserved insertion order as an implementation detail, but the language-level guarantee begins with Python 3.7.
Insertion order is not sorted order. If you need keys in sorted order, explicitly iterate over sorted(d); the Python tutorial shows dictionary iteration and sorting examples.
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How to retrieve an entry by its position
If you genuinely need the entry at a numeric position, first turn the entries into a list, then index that list:
entry = list(scores.items())[1]
print(entry) # ('Linus', 87)Here, 1 is a list position, so it selects the second entry in insertion order. This creates a list of all key–value pairs; it is useful for an occasional positional selection, but unnecessary when the goal is simply to visit every entry. Similarly, list(d) creates a list of the dictionary’s keys in iteration order.
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Common loop errors
- Using the counter as a key:
enumerate()supplies a traversal number. Use the actual key for dictionary lookups. - Unpacking items incorrectly: each result from
enumerate(d.items())has the shape(counter, (key, value)), so unpack it asfor i, (key, value) in .... - Expecting alphabetical order: dictionaries preserve insertion order in Python 3.7 and later; use
sorted(d)when sorted keys are required. - Confusing a missing key with a position: subscription checks for a key and raises
KeyErrorwhen it is absent;d.get(key, default)supplies a fallback. - Depending on insertion order in older Python versions: the language-level guarantee begins in Python 3.7. The Python Functional Programming HOWTO also describes dictionary iteration over keys, values, and key–value pairs.
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