---
tags: Python
---
# Python Self-Check 7
CS 1358 Introduction to Programming in Python
Fall Semester 2022
Prof. Pai H. Chou
Self-Check 1
Due Date: --
Answer the following questions to check your understanding of your material. Expect the
same kind of questions to show up on your tests. For this course, we use vi and vim
interchangeably.
## 1. Definitions and Short Answers - exceptions
1. If your program tries to print a variable that has not been defined, what kind of exception do you get?
```
NameError
```
2. What kind of exception do you get when you try z = 10 / 0?
```
ZeroDivisionError
```
3. What is the same between ZeroDivisionError and OverflowError?
```
related to number operate
```
4. What kind of exception do you get when you run
```
L = "hello world"
print(L['5'])
?Why?
```
```
Typeerror
'5' is a str not int
```
5. Consider the following interactive session:
```
>>> int('25')
25
>>> int('0x25')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '0x25'
Even though 0x25 is a valid hex literal in Python, why do you still get ValueError? How do you correctly convert '0x25' into 37?
Hint: type help(int) to get documentation on different ways of using the int() function.
```
```
if you omit the second parameter of int function, int() will assume base-10
int('0x25', 16)
```
6. Which of the following expressions cause exceptions (and of what kind), assuming L = "hello"
```
L[4] //'o'
L[0] //'h'
L[5] //IndexError
L[-2] //'l'
L[-5] //'h'
L[-7] //IndexError
```
7. Suppose D = {'Sun': 0, 'Mon': 1, 'Tue': 2, 'Wed': 3}, which of the following expressions or assignment statements cause exceptions and of what kind?
```
D['Sun'] //0
D[2] //Keyerror
D['Thu'] //Keyerror
D['Fri'] = 5 //
```
8. When you try to open a file by fh = open('filename', 'r') but cannot, what kind of exception do you get?
```
OSError
```
9. When trying to open a file as in the previous question, how can your program check if an exception has occurred and inform the user by printing
'Cannot open file' to the standard output and continue running the rest of the program as usual?
```
try:
fh = openfile("filename")
except OSError:
print("Cannot open file")
fh.close
```
10. Suppose you are trying to execute the following sequence of statements
```
1 filenames = ['alpha', 'beta', 'gamma']
2 filenum = input('select a file by typing 1, 2, or 3:')
3 i = int(filenum)
4 fh = open(filenames[i], 'r')
```
```
On which lines can exceptions occur and what types?
//4 Indexerror ValueError
How do you rewrite the code to check and handle all types of exception the same way by printing 'An error has occurred'?
//
filenames = ['alpha', 'beta', 'gamma']
filenum = input('select a file by typing 1, 2, or 3:')
i = int(filenum)
try:
fh = open(filenames[i], 'r')
except:
print("An error has occurred")
How do you rewrite the code to check each type of exception and print an error message for each specific exception?
//
filenames = ['alpha', 'beta', 'gamma']
filenum = input('select a file by typing 1, 2, or 3:')
i = int(filenum)
try:
fh = open(filenames[i], 'r')
except (OSError, Indexerror, OSError) as err:
print(f"{err}")
```
11. Given the following program
```
1 try:
2 x = int(input('enter num1:'))
3 y = int(input('enter num2:'))
4 z = x / y
5 except ValueError:
6 z = 0
7 except ZeroDivisionError:
8 z = x
9 print(z)
```
```
If an exception occurs on line 2, what lines of code are executed next?
//5
If an exception occurs on line 3, what lines of code are executed next?
//5
If an exception occurs on line 4, what lines of code are executed next?
//7
If line 6 is execute, is it possible that lines 7-8 are also executed immediately after?
//no
If either line 6 or line 8 is executed, does line 9 also get executed next?
//no
```
12. Given the following program
```
1 greek = {'alpha': 0, 'beta': 1, 'gamma': 2}
2 try:
3 key = input('enter key alpha, beta, or gamma:')
4 y = input('enter integer: ')
5 z = greek[key] / int(y)
6 except ValueError:
7 print('invalid int')
8 except ArithmeticError:
9 print('arithmetic error')
10 except ZeroDivisionError:
11 print('zero division error')
12 else:
13 print('the value of z is', z)
14 finally:
15 print('last action before leaving try')
```
```
Which line or lines can cause one or more exceptions, of which types, and under what conditions?
//5 indexerror ZeroDivisionError ValueError
In the case of zero division, is line 13 executed? Why or why not? If not, is ZeroDivisionError handled and by which line(s)?
//no handled by except ArithmeticError(line 8)
If there is no exception by the end of line 5, which print statement or statements are executed next?
//12
If the user does not input 'alpha', 'beta', or 'gamma' for the variable key on line 3,
which statements are executed next?
//14
Which statement causes an exception of which type?
//5 Keyerror
What statements are executed after the exception?
//15
Is the exception handled by any of the statements here?
//no
What happens to the exception after the entire code above is finished?
//error to the next level
Suppose you want to modify the code above by handling both KeyError and ValueError exactly the same way by the same print('invalid input') statement, which lines do you modify into what code?
//6
```
13. Failure to open a file causes an OSError, but how can you find out more information about the specific reason why the file cannot be opened?
```
try:
fh = open('myfile')
except OSError as err:
print('error: %s' % str(err))
```
14. Given the following code
```
1 import sys
2 try:
3 try:
4 fh = open('myfile')
5 A = int(fh.read())
6 B = int(fh.read())
7 quotient = A / B
8 except (OverflowError, ZeroDivisionError):
9 quotient = 0.0
10 else:
11 quotient = 1.0
12 finally:
13 print('exiting inner try')
14 print('quotient = %f' % quotient)
15 except OSError as err:
16 sys.stderr.write(str(err))
```
```
If an OSError occurs on line 4, do the following lines get executed? //No
line 11? //No
line 13? //Yes
line 14? //No
line 16? //Yes
```
15. Suppose you are writing a rock, paper, scissors game as follows:
```
1 import sys
2 rps = input('rock, paper, scissors, or quit? [rpsq]')
3 if rps == 'q':
4 sys.exit(0)
5 elif rps in 'rps':
6 play_game(rps)
7 else:
8 # report error in the form of a ValueError exception
```
```
Rewrite the code so that
line 8 reports error in the form of a ValueError exception with an error message,
enclose lines 1-8 in a try-except construct to catch the exception, and
handle the exception by writing the error message to sys.stderr.
```
```
import sys
rps = input('rock, paper, scissors, or quit? [rpsq]')
try:
if rps == 'q':
sys.exit(0)
elif rps in 'rps':
play_game(rps)
else:
raise ValueError("ValueError")
except ValueError as err:
sys.stderr.write(str(err))
```
16. Rewrite the code in the previous problem by using assertion instead. This means
replace lines 5-8 with an assert condition and the error message,
enclose the code in a try-except construct but catch the assertion type of exception (what is it?) instead of ValueError. Handle it by writing the error message to sys.stderr also.
```
import sys
rps = input('rock, paper, scissors, or quit? [rpsq]')
try:
if rps == 'q':
sys.exit(0)
assert rps in 'rps','not rps'
except AssertionError as err:
sys.stderr.write(str(err))
```
## 2. Definitions and Short Answers - files
1. When opening a file using fh = open('filename'), why is it ok to omit the second parameter?
```
if omit the second parameter it defaults to reading
```
2. What is the difference between opening a file with 'w' mode vs 'a' mode, as in
fh = open('filename', 'w') vs. fh = open('filename', 'a')?
```
fh = open('filename', 'w'):wipe out ('truncate') if exists; else create
fh = open('filename', 'a'):append if exists; else create
```
3. Once you opened a file as file handle named fh, how do you
```
read one character at a time as a str
//fh.read(1)
read 10 characters as a str
//fh.read(10)
read one line as a str
//fh.readline()
read all the lines as a list of str
//fh.readlines()
read the entire file as one str
//fh.read()
```
4. When reading a file either one line at a time or a number of characters at a time, when do you know you have reached the end of the file?
```
if it return a empty string
```
5. To open a file named 'myfile' for writing (or overwrite it completely if already exists), how should you open the file?
```
fh = open("myfile", 'w')
```
6. Once a file has been opened as file handle fh for writing, how should you write a string 'hello' to it?
```
fh.write("hello")
fh.close()
```
7. What is the difference between print(s) and sys.stdout.write(s)?
```
print(s):s can be a lot of data
sys.stdout.write(s):s only can be str
```
8. When you are done with a file referenced by file handle fh, how do you close it?
```
fh.close()
```
9. Convert the following code into one that uses the with construct. What are some advantages?
```
1 fh = open('filename', 'r')
2 print('totally %d lines in file' % len(fh.readlines()))
3 fh.close()
```
```
with open('filename', 'r') as fh:
print('totally %d lines in file' % len(fh.readlines()))
with construct automatically closes the file
```
10. If you finish reading a file (whose handle is fh) but want to start from beginning again, what should you do without closing and reopening the file?
```
fh.seek(0)
```
11. After you have opened a file whose file handle is fh for reading or writing for a while, how do you find the current position in the file?
```
fh.tell()
```
12. What is a difference between input('') and sys.stdin.readline()?
```
input() is a high level of sys.stdin.readline()
```
13. In a Unix-like shell such as bash (not Python shell), what do the following do?
```
$ grep return *.py > result
//find the line which with return of all .py file and write in to result file
$ grep return *.py >> resfile
//find the line which with return of all .py file and append to result file
```
14. In a Unix-like shell, what is the difference between the commands
```
$ wc -w filename
and
$ wc -w < filename
?
```
```
$ wc -w filename will print number of words and filename
$ wc -w < filename will print number of words without filename
```
15. What does the following Unix shell command do?
```
$ grep return *.py | wc
```
```
find the line with return in all .py file and fed into wc, count lines, words, characters
```
16. Why should you write to sys.stderr instead of sys.stdout to display a text-based error message, even though both appear on the same text terminal?
```
when you redirects error message to a file, you didn`t get the output
```
17. If you open a text file fh, the data object returned by fh.read() and the parameter s passed to fh.write(s) are of str type. If you open a binary file bh, what is the data type of the data object returned by bh.read() and parameter s passed to bh.write(s)?
```
bytes object
```
18. How do you express the literal for a bytes data object consisting of ASCII characters 'h', 'e', 'l', 'l', 'o'?
```
b'hello'
```
19. If you want to convert a bytes literal b'world' into a str type object, why can't you just do str(b'world') even though that is how you would convert other types of objects into str, such as str(23), str(['a', 'b', 'c'])? What is the proper way?
```
you need to set up the enconding because bytes literal can not be a byte
str(b'world', 'UTF8')
```
20. How do you convert from a str object denoted by textstring into bytes type?
```
bytes(str, 'UTF8')
```
21. What is the meaning of the bytes literal b'\xe4\xbd\xa0\xe5\xa5\xbd'?
```
你好
```
22. In Python, what module and what function can be called to get the path to the current working directory? How would you write a short program to print it?
```
os moudle getcwd() function
import os
print(os.getcwd())
```
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