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CS2021
Week 2
Off and Running with Python
Two ways to run Python
• The Python interpreter
– You type one expression at a time
– The interpreter evaluates the expression and prints its
value. No need for print statements.
• Running a Python program
– Python evaluates all the statements in the file, in
order
– Python does not print their values (but does execute
print statements)
• Writing an expression outside a statement (assignment,
print, etc.) is useless, unless it is a function call that has a
side effect
The Python interpreter
The interpreter is a loop that does:
– Read an expression
– Evaluate the expression
– Print the result
If the result is None, the interpreter does not print it
This inconsistency can be confusing!
(Jargon: An interpreter is also called a “readeval-print loop”, or a REPL)
How to launch the Python interpreter
Two ways to launch the interpreter:
– Run IDLE; the interpreter is called the “Python shell”
– Type python at the operating system command line
• Type exit() to return to the operating system command line
Python shell and OS shell are not the same:
• Operating system command line, or “shell” or “command
prompt” (cmd.exe under Windows) or “terminal”
– Runs programs (Python, others), moves around the file system
– Does not understand Python code like 1+2 or x = 22
• Python interpreter
– Executes Python statements and expressions
– Does not understand program names like python or cd
Interpreter: Command History
• Common to re-evaluate statements, or modify
past statements
• Use command history to cycle back to previous
commands
• Key bindings defaults in IDLE Preferences-Keys
– Classic bindings for both OSX and Windows
• history-previous is <Control-p>
• history-next is <Control-n>
Two ways to Run a Python program
• Python evaluates each statement one-by-one
• Python does no extra output, beyond print
statements in the program
• Two ways to run a program:
– While editing a program within IDLE, press F5 (menu
item “Run >> Run Module”)
• Must save the program first, if it is modified
– Type at operating system command line:
python myprogram.py
Python interpreter vs. Python program
• Running a Python file as a program gives different results
from pasting it line-by-line into the interpreter
– The interpreter prints more output than the program would
• In the Python interpreter, evaluating a top-level expression
prints its value
– Evaluating a sub-expression generally does not print any output
– The interpreter does not print a value for an expression that
evaluates to None
• This is primarily code that is executed for side effect: assignments,
print statements, calls to “non-fruitful” functions
• In a Python program, evaluating an expression generally
does not print any output
What are Modules?
• Modules are files containing Python
definitions and statements (ex. name.py)
• A module’s definitions can be imported
into other modules by using “import name”
• The module’s name is available as a global
variable value
• To access a module’s functions, we use dot
notation and type “name.function()”
More on Modules
• Modules can import other modules
• Each module is imported once per interpreter
session
– Multiple import statements are ignored.
However, reload(name) will re-run execution
• We can also directly import names from a module
into the importing module’s symbol table
– from mod import m1, m2 (or *)
– If m1 is a function, then call it with m1()
Executing Modules
• python name.py <arguments>
– Runs code as if it was imported
– Setting _name_ == “_main_” the file can be
used as a script and an importable module
Example: person.py
class Person:
def __init__(self, name, age, pay=0, job=None):
self.name = name
self.age = age
self.pay = pay
self.job = job
def lastName(self):
return self.name.split()[-1]
if __name__ == '__main__’:
bob = Person('Bob Smith', 42, 30000, 'software’)
sue = Person('Sue Jones', 45, 40000, 'hardware’)
print(bob.name, sue.lastname())
$ python person.py
('Bob Smith', 'Jones')
The Module Search Path
• When import name is executed the interpreter
searches for a file named name.py in several
locations
– Current working directory given by os.getcwd()
– System path given by variable sys.path
– sys.path will include a list of directories specified
by environment variable called PYTHONPATH
• Script being run should not have the same
name as a python standard library.
sys.path
• A list of strings that specifies the search path for modules.
Initialized from the environment variable PYTHONPATH,
plus an installation-dependent default which includes the
directory of all installed packages.
• As initialized upon program startup, the first item of this
list, path[0], is the directory containing the script that was
used to invoke the Python interpreter. If the script directory
is not available (e.g. if the interpreter is invoked
interactively or if the script is read from standard input),
path[0] is the empty string, which directs Python to search
modules in the current working directory first. Notice that
the script directory is inserted before the entries inserted
as a result of PYTHONPATH.
Example: showing system path
Python 3.5.0b4 (v3.5.0b4:c0d641054635, Jul 25 2015,
16:26:13) [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)]
>>> import sys
>>> sys.path
['', '/Users/fred/Documents',
'/Library/Frameworks/Python.framework/Versions/3.5/lib/python35.zip',
'/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5',
'/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/platdarwin',/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/libdynload',
'/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/sitepackages']
>>> import os
>>> os.getcwd()
'/Users/fred/Documents'
>>>
Ex: changing working directory and path
#Suppose I have directory Preview that contains file initdata.py
>>> import initdata
Traceback (most recent call last):
File "<pyshell#4>", line 1, in <module>
import initdata
ImportError: No module named 'initdata'
>>> os.chdir(‘./Preview’)
#also try sys.path.append(‘./Preview’)
>>> os.getcwd()
'/Users/fred/Documents/Preview'
>>> import initdata # has an attribute db
>>> db
Traceback (most recent call last):
File "<pyshell#9>", line 1, in <module>
NameError: name 'db' is not defined
>>> initdata.db
{'sue': {'job': 'hdw', 'pay': 40000, 'age': 45, 'name': 'Sue Jones'},
'bob': {'job': 'dev', 'pay': 30000, 'age': 42, 'name': 'Bob Smith'}}
“Compiled” Python Files
 If files mod.pyc and mod.py are in the same directory,
there is a byte-compiled version of the module mod
 The modification time of the version of mod.py used to
create mod.pyc is stored in mod.pyc
 Normally, the user does not need to do anything to
create the .pyc file
 A compiled .py file is written to the .pyc
 No error for failed attempt, .pyc is recognized as invalid
 Contents of the .pyc can be shared by different
machines
The dir() Function
• Used to find and list the names a module defines
and returns a sorted list of strings
>>> import mod
>>> dir(mod)
[‘_name_’, ‘m1’, ‘m2’]
• Without arguments, it lists the names currently
defined (variables, modules, functions, etc.)
• Use dir to list names of all built-in functions and
variables
>>> dir(__builtins__)
['ArithmeticError', 'AssertionError', 'AttributeError',
'BaseException',…..
Packages
• Collections of modules within a directory
– Directory must have an special file contained within
named __init__.py
• Client uses “dotted module names” (ex. a.b)
– Submodule b in package a
• Saves authors of multi-module packages from worrying
about each other’s module names
• Python searches through sys.path directories for the
package subdirectory
• Users of the package can import individual modules from
the package
• Ways to import submodules
– import PP4E.Preview.initdata
• Submodules must be referenced by full name
• An ImportError exception is raised when the package
cannot be found
Importing Modules from Packages
This term we will be working with Package associated with
the textbook called PP4E. Notice the __init__.py file
contained in the PP4E directory.
>>> sys.path.append('./PP4E')
>>> import Preview.initdata
>>> Preview.initdata.db
{'sue': {'age': 45, 'job': 'hdw', 'name': 'Sue Jones',
'pay': 40000}, 'tom': {'age': 50, 'job': None, 'name':
'Tom', 'pay': 0}, 'bob': {'age': 42, 'job': 'dev',
'name': 'Bob Smith', 'pay': 30000}}
>>> from Preview.initdata import *
>>> db
{'sue': {'age': 45, 'job': 'hdw', 'name': 'Sue Jones',
USING THE BOOK’s PP4E PACKAGE
There are a number of ways to enable imports from this directory tree,
1) Set or change working directory to one that includes PP4E
2) Change the sys.path by appending the directory containing PP4E
3) Add PP4E container directory to your PYTHONPATH module search-path setting
4) Copy the PP4E directory to your Python installation’s Lib\site-packages standard
library subdirectory.
5) Include PP4E's container directory in a ".pth" path file too; see Learning Python
for more on module search path configuration.
The Lib\site-packages directory is automatically included in the Python module
search path, and is where 3rd-party software is normally installed by pip and
setup.py distutils scripts. If you do copy PP4E into site-packages then the following
should work:
$ python
>>> import PP4E.Gui.Tools.spams
spam
spamspamspam
Errors when running python
The Call Stack
• As functions are called, their names are placed on the stack, and as they
return, their names are removed. The Traceback presents us with the list of
called functions (from the first called to the most recent called [most recent
call last]), telling us the file where the call occurred, the line in that file, and the
name of the function the call was made from if any (otherwise '?'). On the next
line slightly indented it tells us the name of the function called.
Traceback (most recent call last):
File "test.py", line 25, in ?
triangle()
File "test.py", line 12, in triangle
inc_total_height()
File "test.py", line 8, in inc_total_height
total_height = total_height + height
UnboundLocalError: local variable 'total_height' referenced
before assignment
• We see that execution started in the file
test.py and proceeded to line 25, where the
function 'triangle' was called. Within the
function triangle, execution proceeded until
line 12, where the function 'inc_total_height'
was called. Within 'inc_total_height' and error
occurred on line 8.
Carefully Read the Error message
First function that was
called (<module>
means the interpreter)
Traceback (most recent call last):
File "nx_error.py", line 41, in <module>
Second function
print friends_of_friends(rj, myval)
File "nx_error.py", line 30, in friends_of_friends
that was called
f = friends(graph, user)
Call stack or traceback
File "nx_error.py", line 25, in friends
return set(graph.neighbors(user))#
File "/Library/Frameworks/…/graph.py", line 978, in neighbors
return list(self.adj[n])
Last function that
TypeError: unhashable type: 'list'
was called (this one
suffered an error)
List of all exceptions (errors):
http://docs.python.org/2/library/exceptions.html#bltin-exceptions
Two other resources, with more details about a few of the errors:
http://inventwithpython.com/appendixd.html
http://www.cs.arizona.edu/people/mccann/errors-python
The error message:
daunting but useful.
You need to understand:
• the literal meaning of
the error
• the underlying
problems certain
errors tend to suggest
Simple Debugging Tools
print
– shows what’s happening whether there’s a problem or not
– does not stop execution
assert
–
–
–
–
Raises an exception if some condition is not met
Does nothing if everything works
Example: assert len(rj.edges()) == 16
Use this liberally! Not just for debugging!
input
– Stops execution
– (Designed to accept user input, but I rarely use it for this.)
Working with Files and Persistent Data
Files and filenames
• A file object represents data on your disk drive
– Can read from it and write to it
• A filename (usually a string) states where to find
the data on your disk drive
– Can be used to find/create a file
– Examples:
•
•
•
•
"/home/mernst/class/140/lectures/file_io.pptx"
"C:\Users\mernst\My Documents\cute_cat.gif"
"lectures/file_io.pptx"
"cute_cat.gif"
Read a file in python
# Open takes a filename and returns a file.
# This fails if the file cannot be found & opened.
myfile = open("datafile.dat")
# Approach 1:
for line_of_text in myfile:
… process line_of_text
# Approach 2:
all_data_as_a_big_string = myfile.read()
Assumption: file is a sequence of lines
Where does Python expect to find this file (note the relative pathname)?
Two types of filename
• An Absolute filename gives a specific location on disk:
"/home/mernst/class/140/13wi/lectures/file_io.pptx"
or "C:\Users\mernst\My Documents\cute_cat.gif"
– Starts with “/” (Unix) or “C:\” (Windows)
– Warning: code will fail to find the file if you move/rename
files or run your program on a different computer
• A Relative filename gives a location relative to the
current working directory:
"lectures/file_io.pptx" or "cute_cat.gif"
– Warning: code will fail to find the file unless you run your
program from a directory that contains the given contents
• A relative filename is usually a better choice
Reading a file multiple times
You can iterate over a list as many times as you like:
mylist = [ 3, 1, 4, 1, 5, 9 ]
for elt in mylist:
… process elt
for elt in mylist:
… process elt
Iterating over a file uses it up:
myfile = open("datafile.dat")
for line_of_text in myfile:
… process line_of_text
for line_of_text in myfile:
… process line_of_text
# This loop body will never be executed!
Solution 1: Read into a list, then iterate over it
myfile = open("datafile.dat")
mylines = []
for line_of_text in myfile:
mylines.append(line_of_text)
… use mylines
Solution 2: Re-create the file object (slower, but a better choice if the file does not fit in memory)
myfile = open("datafile.dat")
for line_of_text in myfile:
… process line_of_text
myfile = open("datafile.dat")
for line_of_text in myfile:
… process line_of_text
Writing to a file in python
# Replaces any existing file of this name
myfile = open("output.dat", "w")
# Just like printing output
myfile.write("a bunch of data")
myfile.write("a line of text\n")
open for Writing
(no argument, or
"r", for Reading)
“\n” means
end of line
(Newline)
Wrong; results in:
TypeError: expected a character buffer object
myfile.write(4)
myfile.write(str(4))
Right. Argument
must be a string
• GO to Homework #2