This material is based on Google’s Python Class licensed under Creative Commons Attribution 3.0 License, and code samples are licensed under the Apache 2.0 License.
Python is a dynamic, interpreted (bytecode-compiled) language. There are no type declarations of variables, parameters, functions, or methods in source code. This makes the code short and flexible, and you lose the compile-time type checking of the source code. Python tracks the types of all values at runtime and flags code that does not make sense as it runs.
An excellent way to see how Python code works is to run the Python
interpreter and type code right into it. If you ever have a question
like, “What happens if I add an int to a list?” Just typing it into
the Python interpreter is a fast and likely the best way to see what
happens. (See below to see what really happens!)
$ python ## Run the Python interpreter
Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 6
>>> a
6
>>> a + 2
8
>>> a = 'hi'
>>> a
'hi'
>>> len(a)
2
>>> a + len(a) ## try something that doesn't work
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: must be str, not int
>>> a + str(len(a))
'hi2'
>>> foo ## try something else that doesn't work
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined
Note: you may need to type python3 to make sure you are working with the
latest version of python. If you see Python 2.7.9 instead of Python 3.6.7 or later try
that or ask your instructor.
As you can see above, it’s easy to experiment with variables and
operators. Also, the interpreter throws, or “raises” in Python parlance,
a runtime error if the code tries to read a variable that has not been
assigned a value. Like C++ and Java, Python is case sensitive so “a” and
“A” are different variables. The end of a line marks the end of a
statement, so unlike C++ and Java, Python does not require a semicolon
at the end of each statement. Comments begin with a # and extend to
the end of the line.
Python source files use the “.py” extension and are called “modules”.
With a Python module hello.py, the easiest way to run it is with the
shell command
python hello.py Alice
which calls the Python interpreter
to execute the code in hello.py (in the current directory), passing it the command line argument
“Alice”. See the official docs
page on all the different options
you have when running Python from the command-line.
Since Python variables don’t have any type spelled out in the source code, it’s extra helpful to give meaningful names to your variables to remind yourself of what’s going on. So use “name” if it’s a single name, and “names” if it’s a list of names, and “tuples” if it’s a list of tuples. Many basic Python errors result from forgetting what type of value is in each variable, so use your variable names (all you have really) to help keep things straight.
As far as actual naming goes, some languages prefer underscored_parts for variable names made up of “more than one word,” but other languages prefer camelCasing. In general, Python prefers the underscore method but guides developers to defer to camelCasing if integrating into existing Python code that already uses that style. Readability counts. Read more in the section on naming conventions in PEP 8.
As you can guess, keywords like ‘print’ and ‘while’ cannot be used as variable names — you’ll get a syntax error if you do. However, be careful not to use built-ins as variable names. For example, while ‘str’ and ‘list’ may seem like good names, you’d be overriding those system variables. Built-ins are not keywords and thus, are susceptible to inadvertent use by new Python developers.
Now we can look at the different types of python variable we can create in the next section
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