Operation Manual

Experiments in Python

Notes:

# sort a le full of lm names

# dene a function that will return the year a lm was made

# split the right side of the line at the rst ”(”

def lmYear(  l m ):

return  l m.rsplit('(',1)[ 1]

# load the le into a list in Python memory

# and then close the le because the content is now in memory

with open("lmlist", "r") as le:

lmlist = le.read().splitlines()

# sort by name using library function

lmlist.sort()

# sort by year using key to library function - the lm list

# must end with a year in the format (NNNN)

lmlist.sort(key=lmYear)

# print the list

for lm in  lmlist:

print(lm)

Instead of taking input from the user of the program, this experiment takes input

from a ﬁle containing text. It processes the ﬁle to sort its lines into name and year

of release order and then prints the list to the screen.

Getting the data from a ﬁle requires it to be opened, read into a Python list and

then closed. There is a possibility that the ﬁle could be very large and not ﬁt in the

Python memory. We will cover how to solve this problem later.

Sorting the list is easy, as there is a function called “sort”, which runs a well-

established sorting algorithm on the list. It sorts by the initial letters (alphabetically)

of each line. This is done ﬁrst in this example. If you want to sort some other way,

you need to provide the sorting key. This key is the bit of each list entry that is to

be compared with other keys to decide which comes ﬁrst.

The second sort uses a key for the ﬁlm year. To get the year out of any list entry,

the “ﬁlmYear” function splits the entry into two at the rightmost “(” and uses that.

This is the year of the ﬁlm, in our case. This program will fail if the ﬁlm title does

not have a year following the ﬁrst ﬁnal “(” in the name, or if there is a line in the ﬁle

that contains no ﬁlm name. Normally you will have to validate your input. One of

the rules of programming for real people is: “Never trust your user”.

Try removing each sort to see what happens. Try putting extra “(” in some of the

ﬁlm names.