Python 3 Custom Check Migration

Python 3 Custom Check Migration

Only Agent v7+ supports running Python 3 custom checks by default. Upgrade to the latest Agent version to run your Python 3 custom checks natively. Or enable the Python 3 runtime for your Agent v6.14+ if you would like to test your custom checks migration without upgrading your Agent.


This guide provides information and best practices on migrating checks between Python 2 and 3. Use Datadog’s Custom Check Compatibility tool to see whether your custom checks are compatible with Python 3 or need to be migrated.

To provide flexibility in allowing code to run multiple on versions of the Agent, this guide focuses on retaining backwards compatibility.

Editors and tools


Pylint contains functions to help you verify that your custom checks are compatible with Python 3.


Start by installing with pip on Python 2:

$ python2 -m pip install pylint

Replace python2 in the above command if the path to your Python 2 interpreter is different.


Run the pylint command to verify that your custom check or integration runs on Python 3. Replace CHECK with a valid path to a Python module or package folder:

$ python2 -m pylint -sn --py3k CHECK

For example:

$ python2 -m pylint -sn --py3k ~/dev/
************* Module my-check
E:  4, 4: print statement used (print-statement)
W:  7,22: Calling a dict.iter*() method (dict-iter-method)
W:  9, 8: division w/o __future__ statement (old-division)

After addressing the incompatibilities, the same command returns nothing:

$ python2 -m pylint -sn --py3k ~/dev/

While pylint catches any issue that could prevent the Python 3 interpreter from running code at all, it cannot check for logical validity. After code changes are made, make sure to run the check and validate the output.


2to3 converts Python 2 code to Python 3 code. If you have a custom check that is named, run 2to3:

$ 2to3

Running 2to3 prints a diff against the original source file. For more details about 2to3, see the official 2to3 documentation .


Most modern IDEs and editors provide advanced linting automatically. Make sure that they are pointed to a Python 3 executable, so that when you open a legacy Python 2 only file, any linting errors or warnings show up on the side as a colorful tick in PyCharm or as a clickable box on the bottom in Visual Studio Code.

Python migration

Package imports

To standardize Datadog package namespacing, with Python3, all resources live under the base subpackage. For example:

from datadog_checks.checks import AgentCheck


from datadog_checks.base.checks import AgentCheck


Six is a Python 2/3 compatibility library intended to allow developers to ship Python code that works in both Python 2 and Python3. Some of the examples below make use of six to make legacy Python 2 code compatible with Python 3.

Dictionary methods

In Python 3, the dict.iterkeys(), dict.iteritems() and dict.itervalues() methods are not available.

Python 2 Python 2 and 3
for key in mydict.iterkeys():
for key in mydict:
for key, value in mydict.iteritems():
from six import iteritems

for key, value in iteritems(mydict):
for value in mydict.itervalues():
from six import itervalues

for value in itervalues(mydict):

Also, in Python 3, the dict.keys(), dict.items(), dict.values() methods return iterators. Therefore, if the dictionary needs to be modified during iteration, make a copy first. To retrieve a dictionary’s keys/items/values as a list:

Python 2 Python 2 and 3
mykeylist = mydict.keys() mykeylist = list(mydict)
myitemlist = mydict.items() myitemlist = list(mydict.items())
myvaluelist = mydict.values() myvaluelist = list(mydict.values()

The dict.has_key() method is deprecated in Python 2 and is removed in Python 3. Use the in operator instead.

Python 2 Python 2 and 3
mydict.has_key('foo') //deprecated foo in mydict

Standard library changes

Python 3 features a reorganized standard library, where several modules and functions were renamed or moved. Importing moved modules through six.moves works on both Python versions.

Python 2 Python 3 Python 2 and 3
import HTMLParser import html.parser from six.moves import html_parser

Consult the Six documentation for the list of renamed modules. Note: The urllib, urllib2, and urlparse modules have been heavily reorganized.


Python 2 treats Unicode text and binary-encoded data the same, and tries to automatically convert between bytes and strings. This works as long as all characters are ASCII, but leads to unexpected behavior when it encounters non-ASCII characters.

type literal Python 2 Python 3
bytes b'…' binary binary
str ‘…’ binary text
unicode u'…' text text

Text data is Unicode code points; you must encode with .encode(encoding) for storage or transmission. Binary data is encoded code points represented as a sequence of bytes that must be decoded with .decode(encoding) back to text. When reading text from a file, the open function from the io package is handy because the data read is already decoded into Unicode:

from io import open

f = open('textfile.txt', encoding='utf-8')
contents =  # contents will be decoded to unicode using ‘utf-8’; these are not bytes!

Consult Ned Batchelder’s Pragmatic Unicode for further details.


In Python 3, print is explicitly treated as a function; to turn print into a function regardless of the Python version, put from __future__ import print_function at the top of any file using the old print statement and add parentheses to perform the function call.

Python 2 Python 2 and 3
print "foo" from __future__ import print_function


Integer division

In Python 2, the / operator performs floor division on integers.

Python 2

>> 5/2

In Python 3, the / operator performs float division. The // operator performs floor division.

Python 3

>> 5/2
>> 5//2

To replicate the same behavior of Python 3 regardless of the Python version, put from __future__ import division at the top of any file that uses division and use // for flooring division results.


In Python 2 the standard library round method uses the Round Half Up Strategy while Python 3 uses the Round To Even strategy.

Python 2

>> round(2.5)
>> round(3.5)

Python 3

>> round(2.5)
>> round(3.5)

Datadog provides a utility function, round_value, in datadog_checks_base to allow the replication of the Python 2 behavior in both Python 2 and 3.


Python 3 features different syntax for except and raise.

Python 2 Python 2 and 3
except Exception, variable:
except Exception as variable:
raise Exception, args raise Exception(args)

Relative imports

In Python 3, relative imports must be made explicit, using the dot (.) syntax.

Suppose your package is structured like this:


Suppose also that contains a function called gcd—which contains subtleties distinct from the standard library math module’s gcd function—and you want to use the gcd function from your local package, not the one from the standard library.

In Python 2, if you are inside a package, this package’s own modules take precedence before global modules. Using from math import gcd imports the gcd from mypackage/

In Python 3, import forms not starting with . are interpreted as absolute imports. Using from math import gcd imports the gcd from the standard library.

Python 2 Python 2 and 3
from math import gcd from .math import gcd

Or, for extra readability:

Python 2 Python 2 and 3
from math import gcd from mypackage.math import gcd


Several functions in Python 2 that return lists return iterators in Python 3. These include map, filter, and zip.

The simplest fix to retain Python 2 behavior is to wrap these functions with a call to list:

Python 2 Python 2 and 3
map(myfunction, myiterable) list(map(myfunction, myiterable))
filter(myfunction, myiterable) list(filter(myfunction, myiterable))
zip(myiterable1, myiterable2) list(zip(myiterable1, myiterable2))

The xrange function is removed in Python 3; instead, the range function returns an iterable range object. Import range with from six.moves import range.

Use the built-in next function instead of calling the next method. For instance, rewrite as next(iterator).

Further Reading