Create rule S6928: Python side effects should not be used inside a "t…

…f.function" (#3638)

rules/S6928/metadata.json

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+{
+}

rules/S6928/python/metadata.json

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+{
+  "title": "Python side effects should not be used inside a \"tf.function\"",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6928",
+  "sqKey": "S6928",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "MAINTAINABILITY": "MEDIUM",
+      "RELIABILITY": "HIGH"
+    },
+    "attribute": "CONVENTIONAL"
+  }
+}

rules/S6928/python/rule.adoc

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+This rule raises an issue when a Python side effect happens inside a `tensorflow.function`.
+
+== Why is this an issue?
+
+Python sides effects such as printing, mutating a list or a global variable, inside of a `tensorflow.function` may not behave as expected. 
+Because of the https://www.tensorflow.org/guide/function#rules_of_tracing[Rules of tracing],
+the execution of side effects will depend on the input values of the function and will execute only once per tracing.
+
+[source,python]
+----
+import tensorflow as tf
+
+@tf.function
+def f(x):
+  print("A side effect", x)
+
+f(1) # prints "A side effect 1"
+f(1) # does not print anything
+f(2) # prints "A side effect 2"
+----
+
+The example above depicts the issue encountered when using Python side effects in a `tensorflow.function`. As a single trace is created per input values,
+the second call to `f(1)` does not output anything to the console.
+
+The best practice would be to avoid using Python side effects and prefer the usage of the TensorFlow API with functions such as `tf.print` or tf.TensorArray`.
+
+
+== How to fix it
+
+To fix this issue either remove the side effect or use the corresponding TensorFlow function.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,python,diff-id=1,diff-type=noncompliant]
+----
+import tensorflow as tf
+
+@tf.function
+def f(x):
+  print("Printing", x) # Noncompliant print is a side effect
+----
+
+==== Compliant solution
+
+[source,python,diff-id=1,diff-type=compliant]
+----
+import tensorflow as tf
+
+@tf.function
+def f(x):
+  tf.print("Printing", x) # Compliant
+----
+
+== Resources
+=== Documentation
+* TensorFlow Documentation - https://www.tensorflow.org/guide/function#executing_python_side_effects[Executing Python side effects]
+* TensorFlow Documentation - https://www.tensorflow.org/api_docs/python/tf/print[tf.print reference]
+* TensorFlow Documentation - https://www.tensorflow.org/api_docs/python/tf/summary[tf.summary reference]
+* TensorFlow Documentation - https://www.tensorflow.org/api_docs/python/tf/Variable#methods[tf.Variable methods reference]
+* TensorFlow Documentation - https://www.tensorflow.org/api_docs/python/tf/TensorArray[tf.TensorArray reference]
+* TensorFlow Documentation - https://www.tensorflow.org/api_docs/python/tf/data[tf.data reference]
+