Quick Recap of Dict Operations¶

Let us recap some of the important concepts and operations related to dict. We will primarily focus on those operations which are important for aggregations and joins.

dict contains heterogeneous type of elements.
Typically it is used to represent a row in a table or a sheet. But we can also use dict to store the aggregated results based up on a key.
We use dict for aggregations or joins, as we need to do a look up for a given key to update or insert. list is not efficient for lookups.
Each and every element in a dict contains key value pair where key is typically column name.
Here are the common dict operations relevant to aggregrations and joins.
- Adding elements to the dict
- Checking if the key exists
- Getting value for a given key
- Updating value if the key exists
As part of aggregations or joins, the output is typically a dict. It means we will have multiple records in the dict in the form of key value pairs.

order_count_by_date = {}

# Adding elements in dict
order_count_by_date['2014-01-01'] = 1
order_count_by_date['2014-01-02'] = 1
order_count_by_date['2014-01-03'] = 1

order_count_by_date

{'2014-01-01': 1, '2014-01-02': 1, '2014-01-03': 1}

# Checking if element exists in dict
'2014-01-01' in order_count_by_date

True

'2014-01-04' in order_count_by_date

False

# Getting value for a given key
order_count_by_date['2014-01-01']

order_count_by_date['2014-01-04'] # Throws KeyError exception

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-7-7dbb9ecd49d8> in <module>
----> 1 order_count_by_date['2014-01-04'] # Throws KeyError exception

KeyError: '2014-01-04'

order_count_by_date.get('2014-01-01')

order_count_by_date.get('2014-01-04') # Returns None

# Updating value
order_count_by_date['2014-01-01'] = 2

order_count_by_date

{'2014-01-01': 2, '2014-01-02': 1, '2014-01-03': 1}

order_count_by_date['2014-01-01'] = order_count_by_date['2014-01-01'] + 1 # Incrementing the existing value for a given key

order_count_by_date

{'2014-01-01': 3, '2014-01-02': 1, '2014-01-03': 1}

order_count_by_date['2014-01-01'] += 1 # Incrementing the existing value for a given key

order_count_by_date

{'2014-01-01': 4, '2014-01-02': 1, '2014-01-03': 1}

order_count_by_date.update({'2014-01-02': 2}) # Alternate way to update an existing element value in dict

order_count_by_date

{'2014-01-01': 2, '2014-01-02': 2, '2014-01-03': 1}

Mastering Python

Quick Recap of Dict Operations¶