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Python Runtime Environment


This article serves as a thorough introduction to Cloud Engine’s Python runtime environment. To quickly get started with Cloud Engine, see Getting Started With Cloud Engine.

A Python project has to have a and requirements.txt under its root directory for Cloud Engine to correctly identify it as a Python project.

By default, Cloud Engine runs Python projects with WSGI. It will load the module and call the global variable application in it as a WSGI function. Therefore, please ensure that contains a global variable/function/class named application that complies with the WSGI specification:
from flask import Flask
app = Flask(__name__)
def index():
return "hi"
from app import app
application = app

Most popular Python-based web frameworks come with support for WSGI, including Flask, Django, and Tornado. We provide the following Flask- and Django-based boilerplates for you to reference and start your project with:

Run Without WSGI

You can run Python programs on Cloud Engine without using WSGI, or you can host your own WSGI server as well. To do so, create a file named leanengine.yaml and add the following configuration:

run: python

Make sure that your app listens on the port specified by the environment variable LEANCLOUD_APP_PORT to provide HTTP services.

Configure Python Version

Cloud Engine is compatible with pyenv’s .python-version. You can place this file under the root directory of your project to specify the Python version you want to use:


Cloud Engine will install Python with the version specified in this file.

At this time, Cloud Engine only supports CPython. It doesn’t support other Python implementations including PyPy, Jython, and IronPython.

For newly created apps, if you don’t specify a Python version, the latest stable version will be used. For apps created before 9/2/2021, Python `2.7` will be used by default to ensure compatibility.

If you are using pyenv on your local computer, pyenv will follow this file to run your project with the specified Python version. We recommend that you use pyenv on your local computer so that the environment on your local computer will be the same as that on the server. See pyenv’s GitHub repository for more information on how to install pyenv.

Install Dependencies (requirements.txt)

Cloud Engine will install the packages specified in requirements.txt with pip:


We recommend that you specify the major version of your packages using a format like leancloud>=2.9.1,<3.0.0. This prevents incompatible changes introduced by major version bumps from causing problems during the deployments of your application.

Customize Build Process

You can override the default behavior by specifying startup commands (run), dependency installation commands (install), and build commands (build) in leanengine.yaml:

run: echo 'run another command'
- {use: 'default'}
- echo 'install additional dependencies here'
- echo 'overwrite default build command here'

See Reference: leanengine.yaml for more information. Below are a few examples:

Start Projects With uWSGI

run: uwsgi --gevent 5000 --http :3000 --wsgi-file --master --process=${LEANCLOUD_AVAILABLE_CPUS} --disable-log

System Dependencies

You can specify the system dependencies provided by the server-side environment using leanengine.yaml:

- imagemagick

See Reference: leanengine.yaml for a complete list of supported system dependencies.

Build Logs

By default, the logs generated during the build process won’t be printed to the console. If the build process fails, the logs from the last completed step will be printed to the console.

To print the complete build log for debugging, check Print build logs if you are deploying from the dashboard or add --options 'printBuildLogs=true' if you are deploying with the CLI.

System Dependencies

Health Check

Cloud Engine is primarily optimized for web applications. Your app is expected to provide HTTP services through the port specified by the environment variable named LEANCLOUD_APP_PORT. Keep in mind that the app should listen on (all interfaces) instead of which is the default host of many frameworks.

While your app is being deployed, Cloud Engine will check your app every second to see if it has been successfully started. If your app has not been started within the time limit (30 seconds by default), the deployment will be canceled. After your app has been deployed, Cloud Engine will run health checks for your app regularly and automatically restart it if the check fails.

The way the health check works is that Cloud Engine will send an HTTP request to the homepage (/) of your app. If it gets an HTTP 2xx response, your app will pass the health check.

Health check and the Cloud Engine SDK

Cloud Engine will also check /__engine/1/ping which is handled by the SDK. If the SDK is integrated correctly, Cloud Engine will not check the homepage (/) anymore.

If Dashboard > LeanEngine > Manage deployment > Your group > Settings > Cloud functions mode is set to Enable, or if functionsMode in leanengine.yaml is set to strict, Cloud Engine will check if the SDK is integrated correctly. If not, it will consider your app to have failed to start.

Customizing startup timeout (startupTimeout)

The default timeout for your app to start is 30 seconds. You can change it to any value between 15 and 120 seconds with leanengine.yaml:

startupTimeout: 60

Cloud Environment

Custom Domains

Projects deployed to Cloud Engine can only be accessed with domains configured. You can bind domains by going to Dashboard > LeanEngine > Manage deployment > Your group > Settings > Domains.

If you bind a domain that starts with stg- (e.g.,, it will be assigned to the staging environment automatically.

Load Balancer and CDN

All HTTP and HTTPS requests sent to Cloud Engine will go through a load balancer that deals with chores including HTTPS encryption, HTTPS redirection, and response compression. You won’t have to implement features for handling these tasks yourself for the programs hosted on Cloud Engine. Meanwhile, the load balancer brings the following restrictions that your program cannot bypass:

  • Paths starting with /.well-known/acme-challenge/ are used by Cloud Engine to automatically renew certificates. Requests sent to these paths won’t be forwarded to your program.
  • The size of a request header (URL and headers) should be within 64K and each line of the request header should be within 8K.
  • The size of a request (for uploading files) should be within 100M.
  • The timeout for connecting or waiting for a response is 60 seconds.

Getting the Client IP Address

Cloud Engine’s load balancer includes the following information depicting the original request in the HTTP header:

  • X-Real-IP: The original IP address.
  • X-Forwarded-Proto: The original protocol (http or https).
  • Forwarded: Information about the proxy, defined by RFC 7239. It contains the IP address and the protocol.

If CDN is enabled, the information included in the HTTP headers above will be those of the CDN rather than the original request.

With CDN enabled, the following HTTP headers will also be present:

  • X-Forwarded-For: IP addresses separated by a comma. The first one would be the IP address of the original request.

It’s possible that the information included in the HTTP headers above gets counterfeited. Cloud Engine won’t be able to guarantee its authenticity.

Python (Flask):

from flask import Flask
from flask import request

app = Flask(__name__)

def index():
return 'ok'

Python (Django):

def index(request):
return render(request, 'index.html', {})

For Cloud Engine applications deployed within Mainland China, CDN will be enabled by default. To ensure that your application always gets the accurate IP addresses of original requests, consider enabling dedicated IP. You can learn more about the differences between CDN and dedicated IP on Binding Your Domains § Cloud Engine Domains.

HTTPS Redirect

When you bind a custom Cloud Engine domain, you can enable Force HTTPS to have the load balancer redirect HTTP requests to HTTPS while keeping the paths.


Force HTTPS won’t work properly if CDN is enabled. You’ll still need to implement redirect in your project’s code.

CDN Caching

If you resolve your custom domain to the CDN (including the shared domain provided by Cloud Engine), the CDN will cache the responses for the requests it has received. There are some default rules for caching followed by the CDN.

The CDN will cache the response if:

  • The response header contains Last-Modified (this indicates that the resource requested is static; for HTML files, they will be cached for at most 60 seconds).

The CDN will not cache the response if:

  • There is an error with the response (not 2xx).
  • The request is not idempotent (like a POST request).
  • The response header doesn’t contain Last-Modified (this often indicates that the resource requested is dynamic).

The age of the cache for a given file will depend on the file type and the value of the Last-Modified header. The less frequently the file gets modified, the longer the file gets cached. You can override the default behavior by configuring Cache-Control and the CDN will try its best to follow your configurations. For example:

  • You can use Cache-Control: no-cache to prevent the response from being cached.
  • You can use Cache-Control: max-age=3600 to specify the age of the cache (here we set it to be 1 hour).

To prevent your application from being affected by the caching mechanism, consider enabling dedicated IP. You can learn more about the differences between CDN and dedicated IP on Binding Your Domains § Cloud Engine Domains.

Environment Variables

The following environment variables are available for your application to use:

Variable nameDescription
LEANCLOUD_APP_IDThe App ID of the current application.
LEANCLOUD_APP_KEYThe App Key of the current application.
LEANCLOUD_APP_MASTER_KEYThe Master Key of the current application.
LEANCLOUD_APP_ENVThe environment your application is running in. If you are running your application on your local computer, the value will be non-existent or development (if you are starting your application with the CLI). It will be stage for the staging environment and production for the production environment.
LEANCLOUD_APP_PORTThe port opened up for your application. Your application has to listen on this port in order for users to access your service.
LEANCLOUD_API_SERVERThe address used to access the Data Storage service. Please use this value if your application needs to access the Data Storage service or other cloud services with the REST API.
LEANCLOUD_APP_GROUPThe group the instance is located at.
LEANCLOUD_REGIONThe region the application is running in. It will be CN for Mainland China and US for the United States.
LEANCLOUD_VERSION_TAGThe version number of the deployment.

The environment variables starting with LC_ (like LC_APP_ID) used by the older version of Cloud Engine have already been deprecated. Those environment variables will still be present for a while to ensure compatibility but will eventually get removed. If you are still using them in your application, please replace them with those starting with LEANCLOUD_.

You can also set up custom environment variables on the dashboard to store configurations.



See Cloud Engine Platform Features § Viewing Logs for more information on how to view logs and access logs on the dashboard.

Cloud Engine will collect the logs your application has printed to standard output (stdout) and standard error (stderr):

import sys

print('hello') # stdout
print('some error', file=sys.stderr) # stderr
Example for printing logs with Python 2
import sys

print 'hello' # stdout
print >> sys.stderr, 'some error' # stderr

Each line of the logs can contain a maximum of 4096 characters. A maximum of 600 lines of logs can be collected every minute. The logs generated by your application that exceed these limits will be discarded.


The server side uses Beijing Time (UTC+8).

File System

Your application can create temporary files under /home/leanengine and /tmp. The size limit for all the files created by your application is 1 GB.


Each time you trigger a new deployment for your application, Cloud Engine will create a new container for it. Even though you don’t trigger deployments, Cloud Engine will still perform occasional maintenance operations. This means that your application should not treat the file system provided by Cloud Engine as permanent storage.

If the files created by your application bear relatively larger sizes, we recommend that your application always cleans them up once it finishes using them. Creating more files when there are already more than 1 GB files existing might lead to the Disk quota exceeded error. You can trigger a deployment to quickly clean up all the temporary files.

IP Addresses

Some third-party platforms (like Weixin Open Platform) may require that you provide an IP address whitelist. You can obtain the inbound and outbound IP addresses used by Cloud Engine on Dashboard > LeanEngine > Manage deployment > Your group > Settings > Inbound IP and outbound IP.


For Cloud Engine applications deployed within Mainland China, CDN will be enabled by default. Depending on the provider we use, the inbound IP addresses might be changed frequently. To get a fixed inbound IP address for your application, consider enabling dedicated IP.

We will do our best to minimize the frequency of changing the inbound and outbound IP addresses, but there remains the possibility for them to get changed. If you encounter any problems with IP addresses, the first thing you can do is look at the IP addresses displayed on the dashboard and see if they have been changed.

To get a fixed inbound IP address for your application, consider enabling dedicated IP.