How to Append Multiple Values to a Dictionary Key in Python?

Appending Multiple Values to a Dictionary Key

For beginners in Python, working with dictionaries can be challenging, especially when it comes to handling multiple values associated with a single key.

Suppose you have a list of years with corresponding values and want to create a dictionary using the years as keys. However, if a year appears multiple times, you need a mechanism to append the values for that year.

Consider the following input data:

2010
2
2009
4
1989
8
2009
7

Your goal is to construct a dictionary that looks like this:

{
    2010: 2,
    2009: [4, 7],  # Appended value for the same key
    1989: 8
}

To achieve this, follow these steps:

<code class="python">years_dict = {}  # Empty dictionary to store years and values

# Iterate through the list of years and values
for line in list:
    year = line[0]   # Extract the year from the line
    value = line[1]  # Extract the value for the year

    # Check if the year is already in the dictionary
    if year in years_dict:
        # If it is, append the new value to the existing list
        years_dict[year].append(value)
    else:
        # If it's a new year, create a new list and add the first value
        years_dict[year] = [value]</code>

This code will create a dictionary where the years are keys and the associated values are stored in lists. If a year appears multiple times, its values are appended to a list.

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