Python Programming - Files & Exceptions

CS.QIAU - Winter 2020
PYTHONFiles & Exceptions- Session 7
Omid AmirGhiasvand
Programming

File
▸ A file is a common storage unit in a computer.
▸ programs can read from a file or write into a file.
▸ Serious program use file or database to save state.
▸ File Extension
▸ determine which program will be associated with the file.
▸ .docx or .txt or .csv or .tsv or .json
▸ .mp3 or .mkv
▸ .exe or .bin or .dmg or .tar.gz
▸ Type of file supported by Python:
▸ Text file
▸ Binary file

Creating or Opening Text File
ﬁle_handler = open(ﬁle_name, mode)
File handler
object Built-in
function

File Handler Object
▸ The open() function return a file handler object for the file name.
▸ The first argument is a string containing the path and file name to be
opened.
▸ The second argument is also a string describing the way in which the file will
be used.
Access Mode

PATH
▸ The first argument is a string containing the path and file name to be
opened.
▸ Absolute/fully qualified path:
▸ Relative path:
“C:UserAmirDocumentscontact.csv”
“.datasample.csv”
“..datasample.csv”
“....datasample.csv”
Current
Directory
Parent
Directory
2 Directories
above the current
Directory
Windows use and Mac OS and Linux use /

Access Mode
▸ The access mode argument is optional.
▸ ‘r’ will be used if omitted.
▸ Useful Access Modes
Mode Description
‘r’ Opens the file in read only mode.
‘w’ Opens the file for writing. If the file already exists then it will get overwritten. If the file does’t exist it creates a new file
‘a’ Opens the file for appending data at the end of the file automatically. If the file does’t exist it creates a new file
‘r+’ Opens the file for both reading and writing.
‘w+’ Opens the file for reading and writing. If the file already exists then it will get overwritten. If the file does’t exist it creates a new file
‘a+’ Opens the file for reading and appending. If a file already exists, the data is appended. If the file does’t exist it creates a new file.
‘x’ Creates a new file. If the file already exists, the operation fails

▸ Opening files consume system resources.
▸ To save resource it is important to close the file once processing is
completed.
▸ If you do not explicitly close a file, Python’s garbage collector will eventually
destroy the object and close the opened file for you, but the file may have
stayed open for a while.
Closing the File
file_handler.close()
Memory Leak

sample.txt
relative path
current folder
Access
mode
read 1
line and
print it

test.txt in
data folder
relative path in
data folder

With Statement To Open and Close Files
with open(file_name, mode) as file_handler :
statement-1
statement-2
statement-3
Keyword
Programmer
DefinedKeyword
We don’t have to close the file manually.
You can use the
file in this block.

▸ File handler methods
Working With File
Method Syntax Description
read() file_handler.read() used to read the contents of a file
readline() file_handler.readline() used to read a single line in file.
readlines() file_handler.readlines() used to read all the lines of a file as list items.
write() file_handler.write() will write the contents of the string to the file. Add n to add newline
writelines() file_handler.writelines() will write a sequence of strings to the file.
tell() file_handler.tell() returns an integer giving the file handler’s current position within the file
seek() file_handler.seek(offset) is used to change the file handler’s position.

read line 2
read line 3
Original ﬁle has 1
empty line between
paragraphs and we add
one more

Python Programming - Files & Exceptions

split() method split
the text into a list
separated by dot
2 dimensionals
list
It’s one item - A
sentence
dot delimiter
It’s a list - list of
sentences
remove white
space from beginning
and end of “line”

▸ Lightweight text-based data-interchange. Commonly used for
transmitting data in web applications.
▸ http/https protocol
JavaScript Object Notation - JSON
Client Server
http request
http response
Client/Server Architecture

▸ Key-value pair
▸ value data type : string, object, number, boolean and list
JSON Format
{
"article_link": “https://www.huffingtonpost.com/entry/versace-black-code_us_5861fbefe4b0de3a08f600d5",
“headline": "former versace store clerk sues over secret 'black code' for minority shoppers",
"is_sarcastic": 0
}
key value

key
json
value
load() method
load a json ﬁle

jsonlines
this ﬁle is not a
valid json ﬁle

Total
number of
json object
loads()
method load a
string line that is a
json in nature
add json
into a list

▸ There are at least two distinguishable kinds of errors
▸ Syntax Errors
▸ Exceptions
▸ Even if a statement or expression is syntactically correct, it may cause an error
when an attempt is made to execute it. An exception is an unwanted event that
interrupts the normal flow of the program.
▸ User Defined
▸ Built-in
▸ Exception Handling
▸ Handling of exception ensures that the flow of the program does not get
interrupted when an exception occurs which is done by trapping run-time errors.
Exception
Error in Run-time

▸ To handle the exception we can use try-except-ﬁnally block.
Exception Handling
try :
statement-1
statement-2
statement-3
except ExceptionName-1
statement-4
except ExceptionName-2
statement-5
else :
statement-6
ﬁnally:
statement-7
depend on which
exception thrown, one of the
except blocks will be
trigger
associated
except block
associated
except block
try block

open
jsonlines ﬁle
load string line
as a json
ﬁnally block will
execute in all cases
else block execute
when no exception
thrown
No exception

exception
the ﬁle is in
data folder

newly
created .csv ﬁle
camma delimiter
opening two
ﬁles in with
we just need
values and not keys

read csv ﬁle
each line is a list
each line is a list
import csv
module

“ The Way You Do One Thing Is the Way You Do Everything”

Python Programming - Files & Exceptions

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