Python – Check if a given key already exists in a dictionary

dictionarypython

I wanted to test if a key exists in a dictionary before updating the value for the key.
I wrote the following code:

if 'key1' in dict.keys():
  print "blah"
else:
  print "boo"

I think this is not the best way to accomplish this task. Is there a better way to test for a key in the dictionary?

Best Answer

in is the intended way to test for the existence of a key in a dict.

d = {"key1": 10, "key2": 23}

if "key1" in d:
    print("this will execute")

if "nonexistent key" in d:
    print("this will not")

If you wanted a default, you can always use dict.get():

d = dict()

for i in range(100):
    key = i % 10
    d[key] = d.get(key, 0) + 1

and if you wanted to always ensure a default value for any key you can either use dict.setdefault() repeatedly or defaultdict from the collections module, like so:

from collections import defaultdict

d = defaultdict(int)

for i in range(100):
    d[i % 10] += 1

but in general, the in keyword is the best way to do it.

Related Solutions

Python – How to check if a list is empty

if not a:
  print("List is empty")

Using the implicit booleanness of the empty list is quite pythonic.

Python – How to check whether a file exists without exceptions

If the reason you're checking is so you can do something like if file_exists: open_it(), it's safer to use a try around the attempt to open it. Checking and then opening risks the file being deleted or moved or something between when you check and when you try to open it.

If you're not planning to open the file immediately, you can use os.path.isfile

Return True if path is an existing regular file. This follows symbolic links, so both islink() and isfile() can be true for the same path.

import os.path
os.path.isfile(fname)

if you need to be sure it's a file.

Starting with Python 3.4, the pathlib module offers an object-oriented approach (backported to pathlib2 in Python 2.7):

from pathlib import Path

my_file = Path("/path/to/file")
if my_file.is_file():
    # file exists

To check a directory, do:

if my_file.is_dir():
    # directory exists

To check whether a Path object exists independently of whether is it a file or directory, use exists():

if my_file.exists():
    # path exists

You can also use resolve(strict=True) in a try block:

try:
    my_abs_path = my_file.resolve(strict=True)
except FileNotFoundError:
    # doesn't exist
else:
    # exists

Best Answer

Related Solutions

Python – How to check if a list is empty

Python – How to check whether a file exists without exceptions

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