Travis CI: Add flake8 to the testing (#5)

* Travis CI: Add flake8 to the testing 

Add [flake8](http://flake8.pycqa.org) tests to find Python syntax errors and undefined names.

__E901,E999,F821,F822,F823__ are the "_showstopper_" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety.
* F821: undefined name `name`
* F822: undefined name `name` in `__all__`
* F823: local variable name referenced before assignment
* E901: SyntaxError or IndentationError
* E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

* learn-python should not teach the antipattern of bare exceptions

See PEP8 and https://realpython.com/the-most-diabolical-python-antipattern

* noqa F821 - We are intentionally raising NameError

* Two consecutive blank lines before a function definition

* flake8 --max-complexity=10 --> 12

* pylint: disable=broad-except

Author: cclauss

.travis.yml

@@ -5,9 +5,14 @@ python:
 # Install dependencies.
 install:
   - pip install -r requirements.txt
+  - pip install flake8
 
 # Run linting and tests.
 script:
+  # stop the build if there are Python syntax errors or undefined names
+  - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+  # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
+  - flake8 . --count --exit-zero --max-complexity=12 --max-line-length=127 --statistics
   - pylint ./src
   - pytest
 

src/control_flow/test_try.py

@@ -18,6 +18,7 @@ def test_try():
 
     # The try block will generate an error, because x is not defined:
     exception_has_been_caught = False
+
     try:
         # pylint: disable=undefined-variable
         print(not_existing_variable)
@@ -41,6 +42,7 @@ def test_try():
     # You can use the else keyword to define a block of code to be executed
     # if no errors were raised.
     message = ''
+    # pylint: disable=broad-except
     try:
         message += 'Success.'
     except NameError:
@@ -54,8 +56,8 @@ def test_try():
     # error or not.
     message = ''
     try:
-        # pylint: disable=undefined-variable
-        print(not_existing_variable)
+        # pylint: undefined-variable
+        print(not_existing_variable)  # noqa: F821
     except NameError:
         message += 'Something went wrong.'
     finally:

src/data_types/test_lists.py

@@ -195,7 +195,7 @@ def test_del_statement():
     with pytest.raises(Exception):
         # Referencing the name a hereafter is an error (at least until another
         # value is assigned to it).
-        assert numbers == []
+        assert numbers == []  # noqa: F821
 
 
 def test_list_comprehensions():

src/modules/test_modules.py

@@ -39,6 +39,7 @@
 # It can also be used when utilising from with similar effects:
 from fibonacci_module import fibonacci_at_position as fibonacci_at_position_renamed
 
+
 # When a module named spam is imported, the interpreter first searches for a built-in module with
 # that name. If not found, it then searches for a file named spam.py in a list of directories
 # given by the variable sys.path. sys.path is initialized from these locations: