I am working on a data intensive project where I have been using PHP for fetching data and encrypting it using phpseclib. A chunk of the data has been encrypted in AES with the ECB mode -- however the key length is only 10. I am able to decrypt the data successfully.
However, I need to use Python in the later stages of the project and consequently need to decrypt my data using it. I tried employing PyCrypto but it tells me the key length must be 16, 24 or 32 bytes long, which is not the case. According to the phpseclib documentation the "keys are null-padded to the closest valid size", but I'm not sure how to implement that in Python. Simply extending the length of the string with 6 spaces is not working.
What should I do?
I strongly recommend you adjust your PHP code to use (at least) a sixteen byte key, otherwise your crypto system is considerably weaker than it might otherwise be.
I would also recommend you switch to CBC-mode, as ECB-mode may reveal patterns in your input data. Ensure you use a random IV each time you encrypt and store this with the ciphertext.
Finally, to address your original question:

According to the phpseclib documentation the "keys are null-padded to the closest valid size", but I'm not sure how to implement that in Python. Simply extending the length of the string with 6 spaces is not working.

The space character 0x20 is not the same as the null character 0x00.
Key length issue: AES encryption on phpseclib and decryption on PyCrypto