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How to read all HTTP headers in Python CGI script?
When working with Python CGI scripts, you often need to access HTTP headers sent by the client. Apache's mod_cgi automatically converts HTTP headers into environment variables with an HTTP_ prefix.
How Apache Handles HTTP Headers
Apache's mod_cgi sets environment variables for each HTTP request header received. The variables follow a specific pattern:
- All header names get an
HTTP_prefix - Header names are converted to uppercase
- Hyphens are replaced with underscores
For example, x-client-version: 1.2.3 becomes HTTP_X_CLIENT_VERSION.
Reading a Specific Header
To read a specific custom header, use os.environ with the converted header name ?
import os
# Reading a specific header
client_version = os.environ.get("HTTP_X_CLIENT_VERSION", "Not found")
print(f"Client version: {client_version}")
Reading All HTTP Headers
The following CGI script prints all HTTP headers and their values ?
#!/usr/bin/env python3
import os
print("Content-Type: text/html")
print("Cache-Control: no-cache")
print()
print("<html><body>")
print("<h1>HTTP Headers</h1>")
for headername, headervalue in os.environ.items():
if headername.startswith("HTTP_"):
print("<p><b>{0}</b> = {1}</p>".format(headername, headervalue))
print("</body></html>")
Common HTTP Headers
Here are some common HTTP headers and their environment variable names:
| HTTP Header | Environment Variable | Description |
|---|---|---|
User-Agent |
HTTP_USER_AGENT |
Browser/client information |
Accept |
HTTP_ACCEPT |
Accepted content types |
Authorization |
HTTP_AUTHORIZATION |
Authentication credentials |
Content-Type |
HTTP_CONTENT_TYPE |
Request body content type |
Error Handling
Always use os.environ.get() with a default value to handle missing headers gracefully ?
import os
# Safe way to read headers
user_agent = os.environ.get("HTTP_USER_AGENT", "Unknown")
auth_header = os.environ.get("HTTP_AUTHORIZATION", "")
print(f"User Agent: {user_agent}")
if auth_header:
print("Authorization header present")
else:
print("No authorization header")
User Agent: Unknown No authorization header
Conclusion
Apache CGI automatically converts HTTP headers to environment variables with HTTP_ prefix. Use os.environ.get() to safely read header values and handle missing headers gracefully.
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