I followed the example on http://docs.aws.amazon.com/IAM/latest/UserGuide/access_policies_examples.html#iam-policy-example-s3 for how to grant a user access to just one buck
Error : An error occurred (AccessDenied) when calling the PutObject operation: Access Denied
I solved the issue by passing Extra Args parameter as PutObjectAcl is disabled by company policy.
s3_client.upload_file('./local_file.csv', 'bucket-name', 'path', ExtraArgs={'ServerSideEncryption': 'AES256'})
I was able to solve the issue by granting complete s3 access to Lambda from policies. Make a new role for Lambda and attach the policy with complete S3 Access to it.
Hope this will help.