How to iterate over dictionaries using ‘for’ loops in Python?

To iterate over a dictionary in Python using a for loop, you can use the following methods depending on what you want to access: keys, values, or both key-value pairs. Here’s how to do it:

1. Iterate Over Keys Only

By default, iterating over a dictionary loops through its keys:

my_dict = {'a': 1, 'b': 2, 'c': 3}

for key in my_dict:
    print(key)  # Output: 'a', 'b', 'c'

# Explicitly using .keys() (same result)
for key in my_dict.keys():
    print(key)

2. Iterate Over Values Only

Use .values() to loop through values:

for value in my_dict.values():
    print(value)  # Output: 1, 2, 3

3. Iterate Over Key-Value Pairs

Use .items() to loop through keys and values together (most common):

for key, value in my_dict.items():
    print(f"Key: {key}, Value: {value}")

# Output:
# Key: a, Value: 1
# Key: b, Value: 2
# Key: c, Value: 3

Key Notes:

• .items(): Returns a list-like object of tuples (key, value) in Python 3 (or a list in Python 2).
• Unpacking: Use key, value to unpack the tuple directly in the loop.
• Order: Dictionaries preserve insertion order in Python 3.7+. In earlier versions, use collections.OrderedDict.

Example: Modify Values During Iteration

You can update dictionary values while iterating:

for key in my_dict:
    my_dict[key] *= 2  # Double each value

print(my_dict)  # Output: {'a': 2, 'b': 4, 'c': 6}

Edge Cases

• Avoid modifying the dictionary structure (adding/removing keys) during iteration; this may cause errors. Use list(my_dict.keys()) to iterate over a static list of keys if needed.
• Use a dictionary comprehension for efficient transformations:

  doubled = {k: v*2 for k, v in my_dict.items()}

Summary

Method	Purpose	Example
for key in d	Iterate over keys	print(key)
d.values()	Iterate over values	print(value)
d.items()	Iterate over key-value pairs	print(key, value)

Use .items() for most use cases where you need both keys and values!