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Common Http 404,500 problems with python scripts on Apache or Nginx

Using python CGI sometimes leads to small annoyances while trying to get these scripts to run. Sometimes a seemingly correct script does not work as expected, the cause being some small hidden problem that’s difficult to spot.

Some of these potential problems are:

The Python script is not marked as executable. When CGI scripts are not executable most web servers will let the user download it, instead of running it and sending the output to the user. For CGI scripts to run properly on Unix-like operating systems, the +x bit needs to be set. Using chmod a+x your_script.py may solve this problem.
On a Unix-like system, The line endings in the program file must be Unix style line endings. This is important because the web server checks the first line of the script (called shebang) and tries to run the program specified there. It gets easily confused by Windows line endings (Carriage Return & Line Feed, also called CRLF), so you have to convert the file to Unix line endings (only Line Feed, LF). This can be done automatically by uploading the file via FTP in text mode instead of binary mode, but the preferred way is just telling your editor to save the files with Unix line endings. Most editors support this.
Your web server must be able to read the file, and you need to make sure the permissions are correct. On unix-like systems, the server often runs as user and group www-data, so it might be worth a try to change the file ownership, or making the file world readable by using chmod a+r your_script.py.
The web server must know that the file you’re trying to access is a CGI script. Check the configuration of your web server, as it may be configured to expect a specific file extension for CGI scripts.
On Unix-like systems, the path to the interpreter in the shebang

(#!/usr/bin/env python)

must be correct. This line calls

/usr/bin/env

to find Python, but it will fail if there is no /usr/bin/env, or if Python is not in the web server’s path. If you know where your Python is installed, you can also use that full path. The commands whereis python and type -p python could help you find where it is installed. Once you know the path, you can change the shebang accordingly:

#!/usr/bin/python

.
The file must not contain a BOM (Byte Order Mark). The BOM is meant for determining the byte order of UTF-16 and UTF-32 encodings, but some editors write this also into UTF-8 files. The BOM interferes with the shebang line, so be sure to tell your editor not to write the BOM.
If the web server is using mod_python, mod_python may be having problems. mod_python is able to handle CGI scripts by itself, but it can also be a source of issues.

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