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===============================================================
Python Tutorial for Robot Framework Test Library Developers
===============================================================
| Copyright |copy| Nokia Siemens Networks 2008-2012
| Licensed under the `Creative Commons Attribution 3.0 Unported`__ license
__ http://creativecommons.org/licenses/by/3.0/
.. contents:: Table of Contents
:depth: 2
Introduction
============
* This is self learning material to learn how to program using `Python
language`_. The target is to learn enough Python to be able to start
creating test libraries for `Robot Framework`_.
* Earlier programming experience is expected but not absolutely
necessary.
* The main study material for this training is the excellent *Dive Into
Python* book which is freely available for on-line reading,
downloading or printing from http://diveintopython.net. It is
targeted for people who already know how to program but do not know
Python before.
* If you are a novice programmer, it might better to start with `Think
Python`_ book. It is also available for free and its target audience
is people without any earlier programming knowledge.
* `Python Tutorial`_, available at http://python.org and included in
the standard Python installation at least on Windows, is also very
good. Some of the sections in this training refer to it instead of
or in addition to Dive Into Python.
* Python coding style guidelines are specified in PEP-8_. Notice that
the Dive Into Python book uses :code:`camelCaseStyle` instead of the
recommended :code:`underline_style`.
* Another highly recommended style guide, covering many essential
Python idioms and techniques, is *Code Like a Pythonista:
Idiomatic Python* available at
http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html
* The official Python website at http://python.org is a good place to
find more documentation and Python related information in
general.
* If you need information about Jython, the Java implementation of
Python, you can start from http://jython.org.
* *The Definitive Guide to Jython* covers Jython in detail and is
useful especially if you are interested about Jython-Java
integration. It is freely available at http://jythonbook.com.
Getting started
===============
Installation
------------
* Most Linux distributions, OS X, and other UNIX like machines have
Python installed by default, but on Windows you probably need to
install it separately. Installers for different platforms can be
found from http://python.org.
* Robot Framework does not yet support Python 3.x versions and this
tutorial is also based on Python 2.x. Any 2.x version up from 2.3 is
sufficient but the latter versions are recommended.
* It is highly recommended that you configure your system so that you
can run Python from command line simply by typing :cli:`python` and pressing
enter.
- On Windows, and possibly on some other systems, this requires
adding Python installation directory into :var:`PATH` environment
variable. For example `Robot Framework User Guide`_ has
instructions on how to do it in its *Installation* section.
Interactive interpreter
-----------------------
* Open the command prompt and type :cli:`python`. On Windows you can
also start the interpreter by selecting ``Start > All Programs >
Python 2.x``.
* Statements and expressions can be written in the interpreter.
Pressing enter will interpret the line and possible results are
echoed. Try for example:
.. sourcecode:: pycon
>>> 1 + 2
3
* Use :cli:`Ctrl-D` to exit on UNIX like machines and :cli:`Ctrl-Z`
and enter on Windows.
- With Python 2.5 and newer you can exit the interpreter also with
command :code:`exit()`.
* Dive Into Python has some more examples:
http://diveintopython.net/installing_python/shell.html
Python editors
--------------
* Most general purpose text editors (Emacs, VIM, UltraEdit, ...) and
IDEs (Eclipse, Netbeans, ...) can be used to edit Python. There are
also some editors specially for Python.
* The most important editor features are source highlighting and
handling indentation. Make sure your editor of choice supports them
either natively or via Python plugin or mode.
* If you do not know any editor, you can at least get started with
`IDLE`_. It is included in the standard Python installation on
Windows and can be installed also on other systems.
Variables
=========
Basic data types
----------------
* Python has strings, integers, floating point numbers, Boolean values
(:code:`True` and :code:`False`) similarly as most other programming
languages.
* Strings can be enclosed into double or single quotes. Different
quotest do not have any difference like they do for example in Perl.
* Unicode strings have a special syntax like :code:`u"Hyv\\xE4\\xE4
y\\xF6\\t\\xE4!"`. Using Unicode with Python is not covered otherwise
in this tutorial.
* :code:`None` is a special value meaning nothing similarly as
:code:`null` in Java.
* Try at least these on the interpreter:
.. sourcecode:: pycon
>>> 2 * 2.5
5.0
>>> 'This is easy'
'This is easy'
>>> "Ain't it"
"Ain't it"
Declaring variables
-------------------
* All different values can be assigned to variables. Valid characters
in variable identifiers are letters, underscore, and numbers,
although numbers cannot start the variable name.
* A variable needs not to be declared, it starts to exist when a value is
assigned for the first time.
* There is no need to specify the variable type either as the type is
got from the assigned variable automatically.
* Try it out:
.. sourcecode:: pycon
>>> a = 3
>>> a
3
>>> b = 4
>>> a*b
12
>>> greeting = 'Hello'
>>> greeting
'Hello'
>>> greeting.upper()
'HELLO'
* It is even possible to assign multiple variables at once:
.. sourcecode:: pycon
>>> x, y = 'first', 'second'
>>> x
'first'
>>> y
'second'
First program
=============
* Create a file :path:`hello.py` with your editor of choice and write
this content into it:
.. sourcecode:: python
print "Hello, world!"
* Then execute the file on the console like this:
.. sourcecode:: console
python hello.py
* As a result you should get :code:`Hello, world!` printed into the
screen. With Robot Framework keywords such messages would end up
into the log file.
* For more interesting examples see Dive Into Python:
http://diveintopython.net/getting_to_know_python/index.html
Functions
=========
Creating functions
------------------
* Creating functions in Python is super easy. This example uses the
interpreter, but you can also write the code into the previous
:path:`hello.py` file and execute it.
.. sourcecode:: pycon
>>> def hello():
... print "Hello, world!"
...
>>> hello()
Hello, world!
* Note that in Python code blocks must be indented (four spaces is the
norm and highly recommended) and you close the block simply by
returning to the earlier indentation level. Inside a block you must
use the indentation level consistently.
* Notice also that this :code:`hello` function is actually already a
valid keyword for Robot Framework!
* A function with arguments is not that more complicated:
.. sourcecode:: pycon
>>> def hello(name):
... print "Hello, %s!" % name
...
>>> hello("Python")
Hello, Python!
>>> hello("Robot Framework")
Hello, Robot Framework!
* The hard part in this example is string formatting (i.e. :code:`"Hello,
%s!" % name`) which uses similar syntax as for example C language.
More information about it can be found e.g. from Dive Into Python:
http://diveintopython.net/native_data_types/formatting_strings.html
Optional and named arguments
----------------------------
* Functions can have default values for some or all of its arguments:
.. sourcecode:: pycon
>>> def hello(name="World"):
... print "Hello, %s!" % name
...
>>> hello()
Hello, World!
>>> hello("Robot")
Hello, Robot!
* If there are several optional arguments, it is also possible to
specify only some of them by giving their name along with the value
as the example below illustrates. Those arguments that do not have
default values cannot be omitted.
.. sourcecode:: pycon
>>> def test(a, b=1, c=2, d=3):
... print a, b, c, d
...
>>> test(0)
0 1 2 3
>>> test(0, 42)
0 42 2 3
>>> test(1, c=10)
1 1 10 3
>>> test(2, c=100, d=200)
2 1 100 200
>>> test(b=0)
Traceback (most recent call last):
File "", line 1, in
TypeError: test() takes at least 1 non-keyword argument (0 given)
* Robot Framework keywords can have default values but they are always
used with positional arguments. For example, if the above :code:`hello`
method was used as a keyword, it could be used with zero or one
argument, and :code:`test` could be used with one to four arguments.
* Dive Into Python explains both optional and named arguments very well:
http://diveintopython.net/power_of_introspection/optional_arguments.html
Variable number of arguments
----------------------------
* Function can also be created so that they take any number of
arguments. This is done by prefixing an argument after required and
optional arguments with an asterisk like :code:`*args`, and it means that
the specified argument gets all the "extra" arguments as a tuple_.
.. _tuple: tuples_
.. sourcecode:: pycon
>>> def example(arg1, arg2, *rest):
... print arg1, arg2, rest
...
>>> example(1, 2)
1 2 ()
>>> example(1, 2, 3)
1 2 (3,)
>>> example(1, 2, 3, 4, 5)
1 2 (3, 4, 5)
* Using variable number of arguments works also with Robot Framework
keywords.
* Python tutorial explains everything in this and the prvious section
in detail:
http://docs.python.org/tutorial/controlflow.html#more-on-defining-functions
Returning values
----------------
* Functions can use :code:`return` statement to return values that can be
assigned to variables or passed to other functions:
.. sourcecode:: pycon
>>> def multiply_by_two(number):
... return number * 2
...
>>> result = multiply_by_two(10)
>>> result
20
>>> result = multiply_by_two(multiply_by_two(2))
>>> result
8
* Robot Framework keywords can also return values that can be assigned
to variables and then used as arguments to other keywords.
Documenting functions
---------------------
* In Python functions, as well as classes and modules, are documented with
so called doc strings:
.. sourcecode:: pycon
>>> def hello():
... """Prints 'Hello, world!' to the standard output."""
... print "Hello, world!"
...
* Interestingly the documentation is available dynamically:
.. sourcecode:: pycon
>>> print hello.__doc__
Prints 'Hello, world!' to the standard output.
* Doc strings are covered pretty well in Dive Into Python:
http://diveintopython.net/getting_to_know_python/documenting_functions.html
* Robot Framework has `libdoc.py`_ tool that can generate test library
documentation based on these doc strings. Documenting functions that
are used as keywords is thus very important.
Container data types
====================
* Python has a nice set of container data types built into the
language with a really simple syntax similarly as in Perl and
Ruby. You are going to use them a lot!
* See Dive Into Python for more information and examples than shown
below: http://diveintopython.net/native_data_types
Lists
-----
* A list is an ordered collection of items which you normally access
by index.
* They also have handy methods like :code:`append`, :code:`insert` and
:code:`pop` to access or alter the list.
.. sourcecode:: pycon
>>> x = ['Some', 'strings', 'here']
>>> x[0]
'Some'
>>> x[1]
'strings'
>>> x[-1]
'here'
>>> x[2] = x[2].upper()
>>> x.append(42)
>>> x
['Some', 'strings', 'HERE', 42]
Tuples
------
* A tuple is a list like structure which you cannot alter after creating it.
.. sourcecode:: pycon
>>> t = (1, 2, 'x')
>>> t[0]
1
>>> t[-1]
'x'
>>> t[0] = 'new value'
Traceback (most recent call last):
File "", line 1, in
TypeError: 'tuple' object does not support item assignment
* Notice that you must use a trailing comma to create a tuple with one
element:
.. sourcecode:: pycon
>>> empty = ()
>>> one = (1,)
>>> two = (1, 2)
Dictionaries
------------
* A dictionary is an unordered collection of key-value pairs. The same
data structure is often called hashmap.
.. sourcecode:: pycon
>>> d = {'x': 'some value', 'a': 1, 'b': 2}
>>> d['a']
1
>>> d['x']
'some value'
>>> d['a'] = d['b']
>>> d['tuple'] = t
>>> d
{'a': 2, 'x': 'some value', 'b': 2, 'tuple': (1, 2, 'x')}
>>> 'x' in d
True
>>> 'z' in d
False
Control Flow
============
Conditional execution
---------------------
* Python has similar :code:`if/elif/else` structure as most other
programming languages.
* Notice that no parentheses are needed around the expression as in
Java or C.
.. sourcecode:: python
def is_positive(number):
if number > 0:
return True
return False
def greet(name, time):
if 7 < time < 12:
print 'Good morning %s' % name
elif time < 18:
print 'Good afternoon %s' % name
elif time < 23:
print 'Good night %s' % name
else:
print '%s, you should be sleeping!' % name
Looping
-------
* :code:`for` loops allow iterating over a sequence of items such as
a list. This is probably the loop you are going to use most often.
.. sourcecode:: python
def greet_many(names):
for name in names:
print 'Hello %s' % name
def count_up(limit):
for num in range(1, limit+1):
if num == limit:
print 'bye!'
else:
print num
* :code:`while` loops iterate as long as given expression is true. Very handy
when waiting some event to occur.
.. sourcecode:: python
def wait_until_message_received():
msg = try_to_receive_message()
while msg is None:
time.sleep(5)
msg = try_to_receive_message()
return msg
* Both :code:`for` and :code:`while` loops have typical
:code:`continue` and :code:`break` statements that can be used to
end the current iteration or exit the loop altogether.
* For more examples and information see:
- Python Tutorial: http://docs.python.org/tutorial/controlflow.html
- Dive Into Python: http://diveintopython.net/file_handling/for_loops.html
List comprehensions
-------------------
* Quite often :code:`for` loops can be replaced with even more concise list
comprehensions or generator expressions:
.. sourcecode:: pycon
>>> numbers = [1, -5, 4, -32, 0, 42]
>>> positive = [ num for num in numbers if num > 0 ]
>>> positive
[1, 4, 42]
>>> sum(num * 2 for num in positive)
94
* This syntax might look a bit strange at first but you will love it
very soon. To learn more see, for example, Dive Into Python:
http://diveintopython.net/native_data_types/mapping_lists.html
Modules
=======
Importing modules
-----------------
* Importing existing Python modules is as simply as saying :code:`import
modulename`.
* An alternative syntax is :code:`from modulename import something`.
* Python has a comprehensive `standard library`_ and a `package
index`_ with external modules so there is plenty of existing code to
be imported. It is recommended to study what is available to avoid
reinventing wheels.
Creating modules
----------------
* Because every :path:`.py` file is effectively a Python module, you
have already created at least :code:`hello` module.
* For example if we have the following code in a file called
:path:`example.py`:
.. sourcecode:: python
def hello(name="World"):
print "Hello, %s!" % name
if __name__ == "__main__":
hello()
then we can use it in the interpreter (or from other modules) like:
.. sourcecode:: pycon
>>> import example
>>> example.hello("Tellus")
Hello, Tellus!
* :code:`if __name__ == "__main__"` block in the previous example is
important because it allows executing the file also as a script like
:cli:`python example.py`.
* The automatic :code:`__name__` attribute (Python has many of these
as you will see if you study it more) gets value :code:`"__main___"`
when the file is run as a script and the :code:`if` block is thus
executed only in that case.
* Bigger modules can be organized into several files inside a higher
level module as submodules. In this case the higher level module is
a directory with a special :path:`___init___.py` file.
* For more information about modules see Python Tutorial:
http://docs.python.org/tutorial/modules.html
Module search path (PYTHONPATH)
-------------------------------
* Python modules are not automatically searched everywhere on you
machine. Python has certain default places to search modules for
(e.g. its own library directory which is often in place like
:path:`C:\\Python26\\Lib` or :path:`/usr/lib/python2.6`) and
additionally it looks for them from so called :var:`PYTHONPATH`.
* :var:`PYTHONPATH` is most often controlled using an environment
variable with the same name that contains places (mainly
directories) to look for Python modules. It is similar to Java's
:var:`CLASSPATH` and also to :var:`PATH` environment variable which
is used by an operating system to look for executable programs.
* :var:`PYTHONPATH` is important also with Robot Framework because it
can import test libraries only if the module containing the library
can be imported.
Advanced features
=================
Classes and instances
---------------------
* Python is an object-oriented language but as we have seen you do not
need to use classes everywhere like you need to with Java. It is
totally fine to just have a module with functions if that suites
your needs, but object oriented features are often really handy.
* The syntax for creating classes and then instances from them is
relatively straightforward:
.. sourcecode:: pycon
>>> class MyClass:
... def __init__(self, name):
... self._name = name
... def hello(self, other="World"):
... print "%s says hello to %s." % (self._name, other)
...
>>> c = MyClass('Robot')
>>> c.hello()
Robot says hello to World.
>>> c.hello('Tellus')
Robot says hello to Tellus.
* The only surprising part in the syntax is that every class method
must have :code:`self` as the first argument in the signature. After
you create an instance of the class Python binds the method, and it
also takes care of passing the :code:`self` argument automatically
so you do not use it when calling the method.
* To learn more about classes you can follow an interesting example
from Dive Into Python and/or study detailed information from Python
Tutorial:
- http://diveintopython.net/object_oriented_framework
- http://docs.python.org/tutorial/classes.html
Exceptions
----------
* Python has an exception system similar to many other languages.
Exceptions are classes and the normal way to raise them is
:code:`raise SomeException("Error message")`.
* Exceptions are handled in a :code:`try/except` block:
.. sourcecode:: python
try:
f = open(path)
except IOError, err:
print "Opening file %s for reading failed: %s" % (path, err)
* The :code:`try/except` block can have multiple :code:`except`
branches, an optional :code:`else` to execute if no exception
occurred, and :code:`finally` to execute both when an exception
occurred and when it did not.
* Compared to Java there are some terminology differences
(:code:`raise` vs. :code:`throw` and :code:`except`
vs. :code:`catch`) but the biggest real difference is that there are
no checked exceptions. This means that you do not need to add
:code:`throws SomeException` to methods that may raise an exception.
* More information can be found, for example, from Dive Into Python:
http://diveintopython.net/file_handling/index.html
* Exceptions are an important part of the Robot Framework Library API
because keywords use them to communicate failures to the framework.
Regular expressions
-------------------
* Regular expressions are really handy for processing strings which is
a really common need in test automation.
* Python has a really fast regular expression engine and it uses a
syntax derived from Perl's regexp syntax similarly as Java and many
other languages.
* Dive Into Python contains a good introduction again:
http://diveintopython.net/regular_expressions/index.html
* Notice that Python strings also have many useful methods
(e.g. :code:`startswith`, :code:`find`, :code:`isdigit`) so regexps
are not needed as often as in Perl or Ruby.
Unit testing
============
* Unit testing is important especially when you start having more
code and unit testing your test library code can be a really good
idea.
* Python has several unit testing frameworks. Two of them,
:code:`unittest` and :code:`doctest`, are in the standard
library. The former is immediately familiar for anyone who has used
JUnit or some other xUnit framework and the other is interesting
because it allows using function doc strings for tests.
* Dive Into Python has really good chapters about `unit testing`__,
`test-driven development`__ (TDD), and refactoring__.
__ http://diveintopython.net/unit_testing/index.html
__ http://diveintopython.net/unit_testing/stage_1.html
__ http://diveintopython.net/refactoring/index.html
Writing test libraries
======================
Robot Framework's test library API is really simple. It is explained
fully in `Robot Framework User Guide`_ and this tutorial only covers
the very basic features with an executable example.
Library API basics
------------------
The test library can be either a module or a class. In case of a
module, a keyword will be created for each top-level function in the
module. In case of a class, a keyword will be created for each public
method of the class.
The most important ways keywords can interact with the framework have already
been covered in this tutorial:
* Keyword name maps to the function name (case insensitively and
underscores removed).
* Keywords have same arguments as implementing functions.
* Failures are reported by raising exceptions.
* :code:`print` statement can be used to log messages.
* Values can be returned using the :code:`return` statement.
* Doc strings are used to document keywords.
Executable example
------------------
The example library and associated test data shown below demonstrate
the most important features of the test library API. You can execute
these test cases in your own environment and edit them to test also
other features. A precondition is having Robot Framework installed__,
but then you only need to get `the library`__ and `the data`__, and
run command :cli:`pybot example_tests.tsv`.
__ http://code.google.com/p/robotframework/wiki/Installation
__ ExampleLibrary.py
__ example_tests.tsv
.. sourcecode:: python
./ExampleLibrary.py
.. csv-table:: Simple test cases using keywords from ExampleLibrary
:class: tsv-example
:file: example_tests.tsv
:delim: tab
.. footer:: Generated by reStructuredText_. Syntax highlighting by Pygments_.
.. _Python language: http://python.org
.. _Robot Framework: http://robotframework.org
.. _Think Python: http://www.greenteapress.com/thinkpython/thinkpython.html
.. _Python Tutorial: http://docs.python.org/tutorial
.. _PEP-8: http://www.python.org/dev/peps/pep-0008/
.. _Robot Framework User Guide: http://code.google.com/p/robotframework/wiki/UserGuide
.. _libdoc.py: http://code.google.com/p/robotframework/wiki/LibraryDocumentationTool
.. _IDLE: http://hkn.eecs.berkeley.edu/~dyoo/python/idle_intro/
.. _standard library: http://docs.python.org/lib/lib.html
.. _package index: http://pypi.python.org
.. _reStructuredText: http://docutils.sourceforge.net/rst.html
.. _Pygments: http://pygments.org/