Python retry mechanism

[mickjermsurawong-stripe]

Summary:

• Implement retry mechanism at abstract HTTPClient class, and use delegated request call to each client implementation.
• Only consider errors from requests http-client, but not others due to overwhelming majority of our clients. This design allows future extension the support to other http-clients. (As opposed to urllib3 Retry that is coupled at the execution invocation.
• Retry conditions follow other language implementations (Ruby[0]/Go[1])
  • Retry on 409 resource conflict (both)
  • Retry on timeout/connection (only Ruby)
• Exponential backoff delay follows other language implementations (Ruby[2]/Go[3])
• Added idempotency key only for POST method, and when it is not given in the header

Concerns:

• Not sure where's best to put delay constants in config. Currently made as static class var
• I'm not sure how to access the actual requests module from within test, and that results in mocking kinds of exception classes that we want to handle (existing tests also rely on the similar mechanism)

Motivation:

• Issue Retry mechanism for failing requests #464 and from conversation in PR Implement retry mechanism when using requests #466

[0] Ruby retry conditions
https://github.com/stripe/stripe-ruby/blob/ec66c3f0f44274f885de8d13de5dce2657932121/lib/stripe/stripe_client.rb#L61
[1] Go retry conditions
https://github.com/stripe/stripe-go/blob/442690c3856640e4913163f506f0070561f3368d/stripe.go#L495
[2] Ruby sleep
https://github.com/stripe/stripe-ruby/blob/ec66c3f0f44274f885de8d13de5dce2657932121/lib/stripe/stripe_client.rb#L80
[3] Go sleep
https://github.com/stripe/stripe-go/blob/442690c3856640e4913163f506f0070561f3368d/stripe.go#L511

[mickjermsurawong-stripe]

cc @jonathan-s

[mickjermsurawong-stripe]

cc @stripe/api-libraries

[mickjermsurawong-stripe]

requests errors to retry

# ConnectionError
requests.exceptions.ConnectionError
requests.exceptions.ProxyError
requests.exceptions.ConnectTimeout

# TimeoutError
requests.exceptions.Timeout
requests.exceptions.ReadTimeout
requests.exceptions.ConnectTimeout

requests errors not to retry

# Connection Error
requests.exceptions.SSLError

# ValueError
requests.exceptions.InvalidHeader
requests.exceptions.InvalidSchema 
requests.exceptions.InvalidURL
requests.exceptions.MissingSchema
requests.exceptions.URLRequired

# TypeError
requests.exceptions.StreamConsumedError

# Only when setting response.raise_for_status()
requests.exceptions.HTTPError

# Encode/Decode
requests.exceptions.ChunkedEncodingError
requests.exceptions.ContentDecodingError 

# Redirect-related
requests.exceptions.TooManyRedirects
requests.exceptions.UnrewindableBodyError

# Retry
requests.exceptions.RetryError

http://docs.python-requests.org/en/master/_modules/requests/exceptions/

[ob-stripe]

Looks great @mickjermsurawong-stripe! Thanks for working on this.

[ob-stripe]

Alright, this looks great to me, but the feature is complex enough that I'd like to get another set of eyes on it.

r? @brandur-stripe

[brandur-stripe]

Left a couple minor comments, but thanks for being so thorough on this one! Looks excellent.

ptal @mickjermsurawong-stripe

[mickjermsurawong-stripe]

Added comment/logging
ptal @brandur-stripe

[brandur-stripe]

LGTM! Thanks.

[mickjermsurawong-stripe]

self-approve after lgtm ^

[mickjermsurawong-stripe]

Released in 2.8 fb8117d

[kyleries]

Ended up needing this feature and came across this thread to check out the implementation. Wanted to drop a belated "thank you" for this PR!