MCS 260 Fall 2020
David Dumas
Every value in Python is actually an object: data packaged together with functions that operate on the data.
Functions that are part of an object are called methods.
They are called with a special dot syntax:
>>> L=[1,2,3]
>>> L.append(4) # method of list
>>> s="sharks with lasers"
>>> s.split() # method of str
['sharks', 'with', 'lasers']
Methods already discussed:
| list | |
|---|---|
| .append(x) | add x to end |
| .insert(i,x) | add x at index i |
| .remove(x) | remove first instance of x |
| .pop() | remove and return last element |
| .index(x) | get index of first x in the list |
| dict | |
|---|---|
| .keys() | return iterable of all keys |
| .values() | return iterable of all values |
| .items() | return iterable of key-value pairs |
| str | |
| .split() | split along whitespace, return list |
| .lower() | convert letters to lowercase |
Note: whitespace means consecutive characters that all produce spaces or newlines (tab, space, $\texttt{"\n"}$, ...)
Here are some additional str methods that are useful:
| str | |
|---|---|
| .strip() | remove leading and trailing whitespace |
| .index(x) | search for substring x and return index |
| .upper() | convert to uppercase |
| .isdigit() | Boolean: all characters digits? |
| .isalpha() | Boolean: all characters letters? |
| .isspace() | Boolean: all characters whitespace? |
| .splitlines() | Split along newline characters (returns list) |
| .replace(old,new) | Replace each occurrence of old with new. |
Example: wordstats.py
"""
Compute statistics on words in a given text
MCS 260 Fall 2020 Lecture 11 - David Dumas
"""
text="""
Fresh fruits and fresh vegetables include all
produce in fresh form generally considered as
perishable fruits and vegetables, whether or
not packed in ice or held in common or cold
storage, but does not include those perishable
fruits and vegetables which have been
manufactured into articles of food of a different
kind or character.
""".strip()
print("Reporting on the text:")
print(text+"\n")
print("Found:")
print(len(text.splitlines()),"lines")
# Note chain of str methods
words = text.replace(","," ").replace("."," ").split()
print(len(words),"words")
distinct_words = []
for w in words:
w = w.lower()
if w not in distinct_words:
distinct_words.append(w)
print(len(distinct_words),"distinct words: ")
for w in distinct_words:
print(w,end=" ")
print()
The .join() method of a string $\texttt{s}$ takes an iterable as parameter, and concatenates each element of the iterable with $\texttt{s}$ between them.
>>> s = "+"
>>> L = ["me","laser-sharks","Shakespeare"]
>>> s.join(L)
'me+laser-sharks+Shakespeare'
>>> "".join(L)
'melaser-sharksShakespeare'
Using "".join(iterable) might be the most common case.
.strip(), .split(), .join(), .splitlines(), .replace() are probably the most-used methods for basic string processing.
The .strip() method takes an optional parameter, $\texttt{chars}$ which must be a string. It removes any characters from $\texttt{chars}$ from the start and end.
>>> s = "mathematics"
>>> s.strip("sam")
'thematic'
The .split() method takes an optional parameter, $\texttt{sep}$, which if given is the delimiter (instead of whitespace).
>>> s = "spiders and ticks and mites"
>>> s.split(" and ")
['spiders', 'ticks', 'mites']
The function str() is not a method, but a converter. It converts any Python value to a "reasonable" string representation. It is what print() uses to decide what to display.
>>> str(1.5)
'1.5'
>>> str([1,"fish",2,"fish"])
"[1, 'fish', 2, 'fish']"
>>> str("foo")
'foo'
The statement
import math
loads the math module, after which functions and constants in that module can be used in your code, e.g.
math.sqrt(2) # square root of 2
math.exp(-1) # 1/e
math.sin(0.5*math.pi) # 1.0
The math module includes all common trig functions, logarithms and exponentials, and the constants pi and e. The the math module docs have a full list.
The random module, imported with
import random
includes functions to generate (pseudo)random numbers, e.g.
>>> random.random() # Random float in [0,1)
0.482824082899013
>>> random.randint(1,6) # Random int between 1 and 6 inclusive
5
>>> random.choice( ["Yes", "No", "Maybe"] ) # random item of seq
'No'
>>> L = ["a","b","c","d"]
>>> random.shuffle(L) # IN-PLACE shuffle of a list
>>> L
['a', 'd', 'c', 'b']
Python uses an equation to generate pseudorandom numbers, starting from some initial data, the seed. By default, the seed is computed from the current time.
So the numbers returned are not random at all!
The PRNG can be manually seeded using random.seed(value).
>>> random.seed(42)
>>> random.random()
0.6394267984578837
>>> random.seed(42)
>>> random.random()
0.6394267984578837