Get key name from Python KeyError exception

Question

I would like to get the key name from the Python KeyError exception:

For example:

myDict = {'key1':'value1'}
try:
    x1 = myDict['key1']
    x2 = myDict['key2'] 
except KeyError as e:
    # here i want to use the name of the key that was missing which is 'key2' in this example
    print error_msg[missing_key]

i have already tried this

print e
print e.args
print e.message

my code is inside django view !

if i use ipython for example and try e.arg or e.message it works fine.
but then i try it while inside a django view i get this results:

"Key 'key2' not found in <QueryDict: {u'key1': [u'value']}>" 
("Key 'key2' not found in <QueryDict: {u'key1': [u'value']}>",) 
Key 'key2' not found in <QueryDict: {u'key1': [u'value']}>

while i just want the ‘key2’

Asked By: yossi

||

Source

Answer 1

myDict = {'key1':'value1'}
try:
    x1 = myDict['key1']
    x2 = myDict['key2'] 
except KeyError as e:
    # here i want to use the name of the key that was missing which is 'key2' in this example
    print "missing key in mydict:", e.message

Answered By: Holger

Answer 2

You can use e.args:

[53]: try:
    x2 = myDict['key2']
except KeyError as e:    
    print e.args[0]
   ....:     
key2

From the docs:

The except clause may specify a variable after the exception name (or
tuple). The variable is bound to an exception instance with the
arguments stored in instance.args

Answered By: Ashwini Chaudhary

Answer 3

The exception contains information about the missing key, for example you can print it:

a={"a":1}
try:
    print(a["b"])
except KeyError as e:
    print(e)        #prints "b"

Answered By: lhk

Answer 4

For KeyErrors, the 2nd element (index 1) of sys.exc_info() holds the key:

import sys
try:
    x1 = myDict['key1']
    x2 = myDict['key2'] 
except KeyError:
    print 'KeyError, offending key:', sys.exc_info()[1]

Output:

KeyError, offending key: ‘key2’

Answered By: GreenMatt

Answer 5

As your dictionary is actually a QueryDict, and therefore returns a different error message, you will need to parse the exception message to return the first word in single quotes:

import re
#...
except KeyError as e:
    m = re.search("'([^']*)'", e.message)
    key = m.group(1)

Answered By: Jon Cairns

Answer 6

Simply, the response of the KeyError is the key itself in case you want to make use of it you just need to call the error as it is.

myDict = {'key1':'value1'}

try:
    x1 = myDict['key1']
    x2 = myDict['key2'] 
except KeyError as missing_key:
    # Used as missing_key for readability purposes only
    print(f"Trying to access a <dict> with a missing key {missing_key}")

P.S. This was tested on Python version 3.8.12

Answered By: haddagart

Get key name from Python KeyError exception

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