Python certifi: How to Use SSL Certificate in Python

Python certifi provides Mozilla’s thoroughly curated collection of Root Certificates for validating the trustworthiness of SSL certificates while verifying the identity of TLS hosts. It has been plucked from the requests project.

Python requests library use its own CA file by default or will use the certifi package’s certificate bundle if installed.

Installing Python certifi

To install the python certifi package, you must type the following command.

python3 -m pip install certifi

# OR

pip install certifi

If you have installed the requests library already, there is a 100% chance that the certifi library is also installed, but you have to check it.

So, if you hit the following command, it will either tell us that the requirement is already satisfied or it will install on your machine.

While it’s possible to pass your own CA bundle to Requests to override the default CAs, several third-party packages use Requests under the hood, and there is no way you can tell them to use the custom location for verification.

How to Install Python certifi on Windows

To install certifi Python on Microsoft Windows:

  1. Type cmd in the search bar and hit Enter to open the command line.
  2. Type python3 -m pip install certifi in the command line and hit Enter again. This installs certifi for your default Python installation.
  3. The previous command may not work if you have both Python versions 2 and 3 on your computer. In that case, try the pip3 install certifi command. It is now installed in your system.

How to Install Python certifi on Linux

To install certifi Python on Linux:

  1. First, open the terminal or shell in your Linux OS.
  2. Type python3 -m pip install certifi, and hit Enter.
  3. If it doesn’t work, try using this command: pip3 install certifi or python -m pip install certifi.

Python certifi.where()

The certifi.where() function helps us find the reference of the installed certificate authority (CA) bundle in Python.

import certifi




You can also find the cacert.pem path from the command line using the following command.

 python -m certifi

Browsers and certificate authorities have finalized that 1024-bit keys are unacceptably weak for certificates, particularly root certificates.

For the same reason, Mozilla has removed any weak (i.e., 1024-bit key) certificate from its bundle, replacing it with the equivalent robust (i.e., 2048-bit or higher key) certificate from the same CA.

Note: Certifi does not support any addition/removal or modification of the CA trust store content. 

If you put the additional certificates in the PEM bundle file, you can use these two environment variables to overwrite the default cert stores used by Python OpenSSL and Requests.


However, we can quickly check for this when our scripts start-up up and update the CA bundle automatically with a given CA if necessary.

First, capture your custom CA and save it as the PEM; you can convert it using OpenSSL.

If you only have a .cer, .crt, or .derenSSL.

openssl x509 -inform der -in certificate.cer -out certificate.pem

When you have multiple custom intermediates or roots, you can add them all into a single .pem file when converting them all.

Drag the certificate.pem into the root of your project.

Now, we’re going to try requesting the target URL. In our case, it is a GitHub API, and if we hit the cert error, update the CA bundle in use by Certifi.

import certifi
import requests

  print('Checking connection to Github...')
  test = requests.get('')
  print('Connection to Github OK.')
except requests.exceptions.SSLError as err:
  print('SSL Error. Adding custom certs to Certifi store...')
  cafile = certifi.where()
  with open('certicate.pem', 'rb') as infile:
    customca =
  with open(cafile, 'ab') as outfile:
    print('That might have worked.')


Checking connection to Github...
Connection to Github OK.

How to fix  ModuleNotFoundError: No module named “certifi”

The ModuleNotFoundError: No module named “certifi” exception is raised when either the certifi module is not installed correctly or you forgot to install the certifi package in Python.

To fix the ModuleNotFoundError: No module named “certifi” error in Python, install the certifi library using “python3 -m pip install certifi” or “pip install certifi” in your operating system’s shell or terminal first.

2 thoughts on “Python certifi: How to Use SSL Certificate in Python”

  1. Krunal, I really enjoyed the well explained in-depth knowledge.

    Quick question: Do we need to get root & intermediate certs(base64) along with publick cert value in the .pem file?

  2. Hi Krunal,

    Thank you for your detailed post. It was really helpful in resolving a self signed certificate error that i was getting since some time.

    @Anil: Yes I would suggest that all root & intermediate certificates are taken together in the pem file.


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