CS 35L Fall 19 Section 3 Notes Week 3 Monday
Zhaowei Tan


Python Basics:
1. Installation and package management:
sudo pip install module_name
sudo pip3 install module_name
Note that you might not be able to do that if you do not have root permission
(i.e., unable to ‘sudo’)
2. Using interactive shell or script execution:
python script.py
python3 script.py
3. Modules are important parts of Python, probably one of the major reasons why
people love Python. Use pip to install a new module in your computer. To
import a module into the script:
import module
import module as module_alias
4. Variable types: You don’t have to declare the variable type when you assign it
– instead, Python would automatically know what type the variable is. E.g. the
following all work:
a = “123”
a = 3
a = 4.0
Note that:
1) To transfer from one type to another, use something explicitly like
a = 4.0
a = int(a)
2) In calculation, the Python would not do the auto type transfer for you
a = 4 / 3 # equals 1 in python 2
a = 4 / 3 # equals 1.333 in python 3
3) To check the type, use type(var)
5. Use print (…) to print things to standard output.
Print can recognize any type of the variable, int, float, list, etc.
Python 3 print special: sep and end
print('G','F','G', sep='')
print("Welcome to" , end = ' ')
6. String:
a. Access a char: s[index]; index can be negative, s[start:end] for slicing
b. Split the string: s.split(‘delimiter’)
c. Find substring within a string: s.find(“substring”, start_position).
Returns index, or negative if not found. Other method: replace,
upper, …
d. Concatenate: c = a + b
7. List:
To create a list
my_list = [‘a’, 4, [1, 3]]
The items in Python list are not necessarily be of same length or same type.
Use list[a] to fetch an item. Use list[a:b] to fetch a subset. Index a is inclusive,
but b is exclusive.
Useful list functions: append, extend, index, remove, pop, count…
8. Dictionary
To initiate a list/directory
my_dict = {“name”: “Zhaowei”, “major”: “CS”, “status”: 0}
Access value by key: my_dict[“name”]
Useful dictionary functions: keys, values, items…
9. Tuples: Not mutable. Old tradition of keeping heterogeneous stuffs. Can be
used as key for dictionary.
10. if condition:
do something
You can use and, or, not to connect the different statements.
Use indentation to wrap the block.
To enable else if:
if condition1:
do something
elif condition2:
do something else
else:
do something different
Some special conditions: if key in dict (check if key is in the keys of dict); if
ele in list
11. Loops:
while condition:
do something
or
for i in [list]:
do something for i
Note:
1) Be careful that Python uses indentation to indicate different levels, instead
of using brackets (C) or keywords (Bash). I recommend using spaces
instead of tabs as your indentation for compatibility.
2) You could use range(min, max, step) to generate a number list and use for
loop to iterate. The list generated includes min, but not max (similar to
list[a:b]). We can use a negative step to generate numbers in a reversed
order.
3) To terminate a while/for loop, use break. To skip the current pass of loop,
use continue.
12. Functions in Python:
def func_name(arg1, arg2, …):
do something
return something
Call this function
func_name(arg1, arg2, …)
You don’t have to specify the type for arguments.
13. Python class
class className:
define your variables
define your functions
A special function is called __init__, which will be called after you instantiate
a new variable.
The functions in the class should have an extra argument, self
Inside the function, to call the variable inside the class, use self.varName

When you put double underscore (__) before the variable name, the name of
the variable changes from __varName to _className__varName outside this
class. This is a common practice to declare the variable as a private variable,
however, we could still access this variable using the transferred variable
name. But never do that! The same for function here.
A class example:
class myClass:
__num = 0

def __init__(self, num):
self.__num = 100
def printNum (self):
print self.__num
def _add_people (self, num):
self.__num += num

t = myClass(100)
t. printNum()
t._add_people(10)
print t._myClass__num #don’t do this in reality!
14. Exception handling.
We could use try/except to capture the error and continue the program.
try:
f = open("testfile", "w")
f.write("This is a test file.”)
except IOError:
print "Error: cannot find file or read data"
else:
print "Written content in the file successfully"
f.close()
Note: this can only catch runtime error, not syntax error