Dictionary iteration in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is dictionary iteration in Python?

Dictionary iteration means going through each key or value in a dictionary one by one to use or check them.

beginner

How do you iterate over keys in a Python dictionary?

Use a for loop directly on the dictionary, like for key in my_dict:. This loops over all keys.

beginner

How can you get both keys and values when iterating a dictionary?

Use for key, value in my_dict.items(): to get each key and its value together.

beginner

What does my_dict.values() do in dictionary iteration?

It gives you all the values in the dictionary, so you can loop over just the values without keys.

beginner

Why is dictionary iteration useful in programming?

It helps you check, change, or use each piece of data stored in the dictionary easily, like looking up phone numbers or settings.

Which method lets you loop over both keys and values in a dictionary?

A items()

B keys()

C values()

D all()

The items() method returns key-value pairs for iteration.

What does this code print?
my_dict = {'a':1, 'b':2}
for key in my_dict:
print(key)

A 1 and 2

B Nothing

C 'a' and 'b'

D Error

Looping over a dictionary directly gives its keys.

How do you loop over only the values in a dictionary?

A for val in my_dict.keys():

B for val in my_dict.values():

C for val in my_dict.items():

D for val in my_dict:

The values() method returns all values for iteration.

What type of object does my_dict.items() return?

A String

B List of values

C List of keys

D View of key-value pairs

items() returns a view object showing key-value pairs.

Which of these is NOT a way to iterate a dictionary?

A for i in range(my_dict):

B for key in my_dict:

C for val in my_dict.values():

D for key, val in my_dict.items():

You cannot use range() directly on a dictionary.

Explain how to loop through a dictionary to get both keys and values.

Think about the method that returns pairs.

Why might you want to iterate over just the keys or just the values in a dictionary?

Consider what you need from the dictionary.

Practice

1. What does the following code do?
for key in my_dict:

easy

A. Iterates over the keys of the dictionary

B. Iterates over the values of the dictionary

C. Iterates over the key-value pairs as tuples

D. Raises a syntax error

Solution

1. Step 1: Understand the for loop syntax

   The loop uses for key in my_dict, which by default iterates over dictionary keys.

2. Step 2: Identify what is accessed in each iteration

   Each iteration gives one key from the dictionary, not values or pairs.

3. Final Answer:

   Iterates over the keys of the dictionary -> Option A

4. Quick Check:

   for key in dict = keys [OK]

Hint: for key in dict loops over keys only [OK]

Common Mistakes:

• Thinking it loops over values
• Confusing keys with key-value pairs
• Assuming it raises an error

2. Which of the following is the correct syntax to iterate over keys and values in a dictionary my_dict?

easy

A. for key, value in my_dict.keys():

B. for key, value in my_dict:

C. for key in my_dict.values():

D. for key, value in my_dict.items():

Solution

1. Step 1: Recall dictionary methods for iteration

   To get keys and values, use my_dict.items() which returns key-value pairs.

2. Step 2: Check syntax correctness

   Only for key, value in my_dict.items(): correctly unpacks keys and values.

3. Final Answer:

   for key, value in my_dict.items(): -> Option D

4. Quick Check:

   Use dict.items() for keys and values [OK]

Hint: Use dict.items() to get keys and values together [OK]

Common Mistakes:

• Using my_dict without .items() for key-value pairs
• Trying to unpack keys() or values() which return single elements
• Syntax errors from missing .items()

3. What is the output of this code?

my_dict = {'a': 1, 'b': 2}
for k, v in my_dict.items():
    print(k, v)

medium

A. a 1\nb 2

B. ('a', 1)\n('b', 2)

C. a\nb

D. 1\n2

Solution

1. Step 1: Understand the loop over items()

   The loop unpacks each key-value pair from my_dict.items().

2. Step 2: Analyze the print statement

   It prints the key and value separated by space on each line, so output lines are "a 1" and "b 2".

3. Final Answer:

   a 1\nb 2 -> Option A

4. Quick Check:

   print(k, v) prints key and value separated by space [OK]

Hint: print(k, v) shows key and value separated by space [OK]

Common Mistakes:

• Expecting tuples printed instead of separate values
• Printing only keys or only values
• Confusing output format with list or tuple

4. Find the error in this code:

my_dict = {'x': 10, 'y': 20}
for key, value in my_dict:
    print(key, value)

medium

A. Dictionary keys cannot be unpacked

B. Incorrect print statement syntax

C. Missing .items() after my_dict

D. No error, code runs fine

Solution

1. Step 1: Identify the iteration target

   The loop tries to unpack two variables from my_dict directly, which yields only keys.

2. Step 2: Correct the iteration method

   To unpack key and value, use my_dict.items() instead of just my_dict.

3. Final Answer:

   Missing .items() after my_dict -> Option C

4. Quick Check:

   Use my_dict.items() to unpack key, value [OK]

Hint: Use .items() to unpack key and value in loop [OK]

Common Mistakes:

• Trying to unpack keys only without .items()
• Assuming dict directly yields key-value pairs
• Ignoring error message about unpacking

5. Given the dictionary data = {'a': 0, 'b': 2, 'c': 0, 'd': 4}, which code snippet creates a new dictionary with only keys having non-zero values?

hard

A. new_dict = {k: v for k, v in data if v != 0}

B. new_dict = {k: v for k, v in data.items() if v != 0}

C. new_dict = {k: v for k in data.keys() if data[k] != 0}

D. new_dict = {k: v for v, k in data.items() if v != 0}

Solution

1. Step 1: Understand dictionary comprehension with condition

   We want to keep only items where value is not zero, so use if v != 0 in comprehension.

2. Step 2: Check correct unpacking and syntax

   new_dict = {k: v for k, v in data.items() if v != 0} correctly unpacks key and value from data.items() and filters by value.

3. Final Answer:

   new_dict = {k: v for k, v in data.items() if v != 0} -> Option B

4. Quick Check:

   Use dict.items() and filter with if condition [OK]

Hint: Use dict.items() with if condition in comprehension [OK]

Common Mistakes:

• Forgetting .items() when unpacking key and value
• Swapping key and value in unpacking
• Using keys() without accessing values