Problem: Given a dictionary and a filter condition. How to filter a dictionary by …
- … key so that only those
(key, value)pairs in the dictionary remain where the key satisfies the condition? - … value so that only those
(key, value)pairs remain where the value satisfies the condition?
In this tutorial, you’ll learn four methods and how they compare against each other. It’s loosely based on this tutorial but extends it by many additional examples and code explanations.
If you’re too busy to read this tutorial, here’s the spoiler:
Method 4 — Dictionary comprehension {k:v for (k,v) in dict.items() if condition} is the most Pythonic and fastest way to filter a dictionary in Python.
However, by reading this short 8-minute tutorial, you’re going to learn a lot about the nuances of writing Pythonic code. So keep reading!
Method 1: Simple Iteration to Filter Dictionary
You should always start with the simplest method to solve a problem (see Occam’s razor) because premature optimization is the root of all evil!
So let’s have a look at the straightforward loop iteration method to filter a dictionary.
You start with the following dictionary of names:
names = {1: 'Alice', 2: 'Bob', 3: 'Carl', 4: 'Ann', 5: 'Liz'}
Filter Python Dictionary By Key (Simple Loop)
You want to keep those (key, value) pairs where key meets a certain condition (such as key%2 == 1).
newDict = dict() # Iterate over all (k,v) pairs in names for key, value in names.items(): # Is condition satisfied? if key%2 == 1: newDict[key] = value
Let’s have a look at the output:
print(newDict)
# {1: 'Alice', 3: 'Carl', 5: 'Liz'}
Only the (key, value) pairs where the key is an odd integer remain in the filtered dictionary newDict.
But what if you want to filter the dictionary by a condition on the values?
Filter Python Dictionary By Value (Simple Loop)
To filter by value, you only need to replace one line in the previous code snippet: instead of writing if key%2 == 1: ..., you use the value to determine whether to add a certain (key, value) pair: if len(value)<5: ....
newDict = dict() # Iterate over all (k,v) pairs in names
for key, value in names.items(): # Is condition satisfied? if len(value)<5: newDict[key] = value print(newDict)
# {2: 'Bob', 3: 'Carl', 4: 'Ann', 5: 'Liz'}
Only the dictionary (key, value) pairs remain in the filtered dictionary newDict where the length of the name string value is less than five characters.
Try It Yourself in Our Interactive Cod Shell (Click “run”):
Now, you know the basic method of filtering a dictionary in Python (by key and by value). But can we do better? What if you need to filter many dictionaries by many different filtering conditions? Do we have to rewrite the same code again and again?
The answer is no! Read on to learn about a more generic way to make filtering a dictionary as easy as calling a function passing the dictionary and the filter function.
Method 2: Generic Function to Filter Dictionary
How can you use different filtering functions on different dictionaries without writing the same code again and again? The answer is simple: create your own generic filtering function!
Your goal is to create a function filter_dict(dictionary, filter_func) that takes a dictionary to be filtered and a filter_func to determine for each (key, value) pair whether it should be included in the filtered dictionary.
You start with the following dictionary of names:
names = {1: 'Alice', 2: 'Bob', 3: 'Carl', 4: 'Ann', 5: 'Liz'}
Let’s create the generic filter function!
def filter_dict(d, f): ''' Filters dictionary d by function f. ''' newDict = dict() # Iterate over all (k,v) pairs in names for key, value in d.items(): # Is condition satisfied? if f(key, value): newDict[key] = value return newDict
The function takes two arguments: the dictionary d to be filtered and the function f that decides if an element should be included in the new dictionary.
You create an empty dictionary newDict and decide for all elements of the original dictionary d whether they should be included. To accomplish this, you iterate over each original (key, value) pair and pass it to the function f: key, value --> Boolean. The function f returns a Boolean value. If it evaluates to True, the (key, value) pair is added to the new dictionary. Otherwise, it’s skipped. The return value is the newly created dictionary newDict.
Here’s how you can use the filtering function to filter by key:
Filter Python Dictionary By Key Using Generic Function
If you want to accomplish the same thing as above—filtering by key to include only odd keys—you simply use the following one-liner call:
print(filter_dict(names, lambda k,v: k%2 == 1))
# {1: 'Alice', 3: 'Carl', 5: 'Liz'}
That was easy! The lambda function you pass returns k%2 == 1 which is the Boolean filtering value associated to each original element in the dictionary names.
Similarly, if you want to filter by key to include only even key, you’d do the following:
print(filter_dict(names, lambda k,v: k%2 == 0))
# {2: 'Bob', 4: 'Ann'}
In fact, it’s equally convenient to filter by value using this exact same strategy:
Filter Python Dictionary By Value Using Generic Function
Here’s how you use our function filter_dict to filter by value:
print(filter_dict(names, lambda k,v: len(v)<5))
# {2: 'Bob', 3: 'Carl', 4: 'Ann', 5: 'Liz'}
In the previous code snippet, you filter the dictionary so that only those (key, value) pairs remain where the value has less than five characters.
print(filter_dict(names, lambda k,v: v.startswith('A')))
# {1: 'Alice', 4: 'Ann'}
In this code snippet, you filter the dictionary so that only those (key, value) pairs remain where the value starts with character 'A'.
Try It Yourself in Our Interactive Cod Shell (Click “run”):
But is this the most Pythonic way? Hardly so! Read on to learn about a functional approach to accomplish the same thing with less code!
Method 3: filter() Function on Dictionary
The filter(function, iterable) function takes a function as input that takes one argument (an element of an iterable) and returns a Boolean value whether this element should pass the filter. All elements that pass the filter are returned as a new iterable object (a filter object).
You can use the lambda function statement to create the function right where you pass it as an argument. The syntax of the lambda function is lambda x: expression and it means that you use x as an input argument and you return expression as a result (that can or cannot use x to decide about the return value). For more information, see my detailed blog article about the lambda function.
Filter Python Dictionary By Key Using filter() + Lambda Functions
Here’s how you can filter a dictionary by key using only a single line of code (without defining your custom filter function as in method 2):
# FILTER BY KEY
print(dict(filter(lambda x: x[0]%2 == 1, names.items())))
# {1: 'Alice', 3: 'Carl', 5: 'Liz'}
You may recognize the same filter lambda function lambda x: x[0]%2 == 1 that returns True if the key is an odd integer. Note that this lambda function takes only a single input as this is how the filter() function works (it requires that you pass a function object that takes one argument and maps it to a Boolean value).
You operate on the iterable names.items() that gives you all (key, value) pairs (an element of the iterable is a (key, value) tuple).
After filtering, you convert the filter object back to a dictionary using the dict(...) constructor function.
Another example:
print(dict(filter(lambda x: x[0]%2 == 0, names.items())))
# {2: 'Bob', 4: 'Ann'}
Let’s see how this works for filtering by value:
Filter Python Dictionary By Value Using filter() + Lambda Functions
You can use the same basic idea—filter() + lambda + dict()—to filter a dictionary by value. For example, if you want to filter out all (key, value) pairs where the value has less than five characters, use the following one-liner:
print(dict(filter(lambda x: len(x[1])<5, names.items())))
# {2: 'Bob', 3: 'Carl', 4: 'Ann', 5: 'Liz'}
And a second example:
print(dict(filter(lambda x: x[1].startswith('A'), names.items())))
# {1: 'Alice', 4: 'Ann'}
Try It Yourself in Our Interactive Cod Shell (Click “run”):
So, far so good. But can we do any better? I mean, a single line of code is a single line of code, right? How can we possible improve on that?
Method 4: Filter By Dictionary Comprehension
The best way to filter a dictionary in Python is to use the powerful method of dictionary comprehension.
Dictionary comprehension allows you to transform one dictionary into another one—by modifying each (key, value) pair as you like.
Filter Python Dictionary By Key Using Dictionary Comprehension
The general framework for dictionary comprehension is { expression context }.
expressiondefines how you would like to change each (key, value) pair.contextdefines the (key, value) pairs, you’d like to include in the new dictionary.
Here’s a practical example that filters all (key, value) pairs with odd keys:
print({k:v for (k,v) in names.items() if k%2 == 1})
# {1: 'Alice', 3: 'Carl', 5: 'Liz'}
And here’s an example that filters all (key, value) pairs with even keys:
print({k:v for (k,v) in names.items() if k%2 == 0})
# {2: 'Bob', 4: 'Ann'}
Let’s look at dictionary filtering by value! Is it any different?
Filter Python Dictionary By Value Using Dictionary Comprehension
No! It’s exactly the same:
print({k:v for (k,v) in names.items() if len(v)<5})
# {2: 'Bob', 3: 'Carl', 4: 'Ann', 5: 'Liz'}
This powerful framework is not only fast and easy to understand, it’s also concise and consistent. The filtering criteria is the last part of the expression so that you can quickly grasp how it’s filtered:
print({k:v for (k,v) in names.items() if v.startswith('A')})
# {1: 'Alice', 4: 'Ann'}
Try It Yourself in Our Interactive Cod Shell (Click “run”):
Related tutorials:
- How to Filter a List of Lists in Python?
- Python Lists filter() vs List Comprehension – Which is Faster?
- How to Filter a List in Python?
- How to Filter a List of Dictionaries in Python?
- How to Filter in Python Using Lambda Functions?
All Four Methods for Copy&Paste
Here are all four methods from the tutorial to simplify copy&pasting:
names = {1: 'Alice', 2: 'Bob', 3: 'Carl', 4: 'Ann', 5: 'Liz'} ''' Method 1: Simple For Loop ''' # FILTER BY KEY
newDict = dict() # Iterate over all (k,v) pairs in names
for key, value in names.items(): # Is condition satisfied? if key%2 == 1: newDict[key] = value print(newDict)
# {1: 'Alice', 3: 'Carl', 5: 'Liz'} # FILTER BY VALUE
newDict = dict() # Iterate over all (k,v) pairs in names
for key, value in names.items(): # Is condition satisfied? if len(value)<5: newDict[key] = value print(newDict)
# {2: 'Bob', 3: 'Carl', 4: 'Ann', 5: 'Liz'} ''' Method 2: Custom Function ''' def filter_dict(d, f): ''' Filters dictionary d by function f. ''' newDict = dict() # Iterate over all (k,v) pairs in names for key, value in d.items(): # Is condition satisfied? if f(key, value): newDict[key] = value return newDict # FILTER BY KEY
print(filter_dict(names, lambda k,v: k%2 == 1))
print(filter_dict(names, lambda k,v: k%2 == 0)) # FILTER BY VALUE
print(filter_dict(names, lambda k,v: len(v)<5))
print(filter_dict(names, lambda k,v: v.startswith('A'))) ''' Method 3: filter() ''' # FILTER BY KEY
print(dict(filter(lambda x: x[0]%2 == 1, names.items())))
print(dict(filter(lambda x: x[0]%2 == 0, names.items()))) # FITER BY VALUE
print(dict(filter(lambda x: len(x[1])<5, names.items())))
print(dict(filter(lambda x: x[1].startswith('A'), names.items()))) ''' Method 4: Dict Comprehension ''' # FITER BY KEY
print({k:v for (k,v) in names.items() if k%2 == 1})
print({k:v for (k,v) in names.items() if k%2 == 0}) # FITER BY VALUE
print({k:v for (k,v) in names.items() if len(v)<5})
print({k:v for (k,v) in names.items() if v.startswith('A')})
But how do all of those methods compare in terms of algorithmic complexity and runtime? Let’s see…
Algorithmic Analysis
Before we benchmark those methods against each other, let’s quickly discuss computational complexity.
All four methods have linear runtime complexity in the number of elements in the original dictionary to be filtered—assuming that the filtering condition itself has constant runtime complexity (it’s independent of the number of elements in the dictionary).
| Method | Complexity |
|---|---|
| Method 1: Loop | O(n) |
| Method 2: Custom | O(n) |
| Method 3: filter() | O(n) |
| Method 4: Dictionary Comprehension | O(n) |
But this doesn’t mean that all methods are equally efficient. The dictionary comprehension is usually fastest for these types of filtering operations because it doesn’t use an intermediate function filter(). And it’s also easiest to understand. Therefore, you should use dictionary comprehension in your own code to filter a dictionary by key or by value!
Where to Go From Here?
Enough theory, let’s get some practice!
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