Stop Using dict[key] to Access Values in Python Dictionaries!

# Reducing the chance of crashing your Python script

We probably learnt to use dict[key] to access a value in a dictionary at the start. But here’s a better way to do that.

The original dict[key] method

d = {'apple':4, 'orange':5, 'pear':6}

x = d['apple']    # 4
y = d['orange']   # 5
z = d['pear']     # 6

We use d[key] to access the value of key in a dictionary d.

a = d['pineapple']  # KeyError

However, if a key doesn’t exist in the dictionary, the d[key] raises a KeyError, which crashes our Python script. Which can get really annoying quickly.

A Sad True Story

So I was running a web scraping script a couple years ago.

Estimated time of completion: Overnight (many many pages)

So I went to sleep, hoping it’ll be done by tomorrow.

It crashed due to a KeyError

The KeyError was from some dumb unimportant part with dict[key]

I just wasted 8 hours of my life due to dict[key]

Using dict.get(key) instead!!

d = {'apple':4, 'orange':5, 'pear':6}

x = d.get('apple')      # 4
y = d.get('orange')     # 5
z = d.get('pear')       # 6
a = d.get('pineapple')  # None

When we use dict.get(key) instead of dict[key]:

• they have the same behaviour if the key exists in the dictionary
• dict[key] raises a KeyError if the key doesn’t exist. Code crashes.
• dict.get(key) returns None if the key doesn’t exist. No crashes.

Or use dict.get(key, defaultValue)

Use if you want to set a default value instead of None

d = {'apple':4}

x = d.get('apple')          # 4
y = d.get('orange', 100)    # 100
z = d.get('pear', 3.14159)  # 3.14159

When we use dict.get(key, defaultValue)

• if key exists, the actual value will be returned
• if key doesn’t exist, defaultValue will be returned

Conclusion

Hopefully this was helpful!

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