I wrote a function, which returns the value to the main code only when the input is given in the correct format, to add in my school project but I am facing a small problem and I can't understand the reason why.
This is the code -
def xinput(text, pattern):
myinput = input(text)
match = re.search(pattern,myinput)
if match is None:
print("\nError. \nPlease give input in correct format.\n")
xinput(text, pattern)
else:
return myinput
val = xinput("Enter a number : ", pattern=r"^[\d]+$")
print(val)
Entering the correct value the first time works but if a wrong input is given the first time and the correct input the second time, it returns None.
Here is the output -
Enter a number : q
Error.
Please give input in correct format.
Enter a number : 1
None
I've checked it and myinput remains 1 till just before returning it.
You need to return the value returned by your recursive calls:
if match is None:
print("\nError. \nPlease give input in correct format.\n")
return xinput(text, pattern) # added return here
else:
return myinput
Since you don't return it in your code, when your function completes it comes inside your if statement and does nothing with the result hence returns None.
Related
I have a python program where it prompts a user input for position or index and deletes the element in the list based on the position or index. The python program works but I'm having issues with the condition where if no user input is given, it automatically deletes the whole line in the list.
Example:
lst = [1,2,3,4,5]
enter position: 2
output: [1,2,4,5]
enter position: #user just pressed enter without giving any input
output: []
I'm writing the function within a class whereby:
def delete(self,index):
"""
This function deletes an item based on the index
:param self: the array
:param index: the index of an item in the array
:return: the array is updated
:raises: IndexError if out of range
"""
if not index:
self.__init__()
if index<0:
index = index + self.count
for i in range(index, self.count -1):
self._array[i] = self._array[i+1]
self.count-=1
and prompting the user input is as such:
position = int(input("Enter position:"))
it's not possible to just press 'enter' without receiving an error due to the position only receiving integers hence I'm looking for a method where if the user doesn't give any position, it registers it and prints just an empty list instead of an error message.
What you're looking for is the try-except block. See the following for an example:
input_invalid = true
while input_invalid:
user_input = input("Enter position: ")
try:
user_input = int(user_input)
input_invalid = false
except ValueError:
print("Please enter a valid integer!")
Here, the try-except block catches any errors (of the type specified
in except) thrown within the code block. In this case, the error results from trying to call int() on a string that does not contain an integer (ValueError). You can use this to explicitly prevent the error and control the logic flow of your program like shown above.
An alternate solution without using try-except is to use the .isdigit() method to validate the data beforehand. If you were to use .isdigit() (which I personally think is better), your code would look something like this:
input_invalid = true
while input_invalid:
user_input = input("Enter position: ")
if user_input.isdigit():
input_invalid = false
else:
print("Please enter a valid integer!")
Hope this helped!
I have this function below, which I have done something wrong in somewhere.
def quantityFunction(product):
valid = False
while True:
if product is not None:
quantity = input("Please enter the amount of this item you would like to purchase: ")
for i in quantity:
try:
int(i)
return int(quantity)
valid = True
except ValueError:
print("We didn't recognise that number. Please try again.")
#If I get here, I want to loop back to the start of this function
return True
return False
To run through, the function is called from the main part of the program like so: quantity = quantityFunction(product)
The return False at the bottom of the code is to do with if product is None, which is needed after a bit of code in another function but has had to go in this function.
If the user input for quantity is a number, all works fine. If it is anything else, the Value Error is printed and you can enter another input. If you put another letter etc in, it repeats again, if you put a number in, it accepts it.
However, it does not return the number you inputted after the letters. It just returns 0.
I suspect this is something to do with how I am repeating the code, i.e. the code should loop back to the start of the function if it hits the Value Error.
Any Ideas?
You said:
the code should loop back to the start of the function if it hits the Value Error.
Then you should not use return statements, otherwise the function will terminate, returning True or False.
Few issue:
1) return statement returns control to the calling function.
2) You are looping over the input, which is wrong.
3) valid=True isn't executed at all.
def quantityFunction(product):
valid = False
while True:
if product is not None:
quantity = raw_input("Please enter the amount of this item you would like to purchase: ")
try:
return int(quantity)
#valid = True (since it is never run)
except ValueError:
print("We didn't recognise that number. Please try again.")
#If I get here, I want to loop back to the start of this function
#return True
return False
quantityFunction("val")
Note : Use raw_input() in case of Python 2.7 and input() in case of 3.x
Try this (some formatting included too, but the functionality should be the same):
def determine_quantity(product): # descriptive function name
if not product: # avoiding nesting
return False
while True:
quantity = input("Please enter the amount of this item you would like to purchase: ")
try:
return int(quantity) # try to convert quantity straight away
except ValueError:
print("We didn't recognise that number. Please try again.")
# nothing here means we simply continue in the while loop
Ideally, you'd take product out. A function should do as little as possible, and this check is better off somewhere else.
def determine_quantity():
while True:
quantity = input("Please enter the amount of this item you would like to purchase: ")
try:
return int(quantity)
except ValueError:
print("We didn't recognise that number. Please try again.")
First, let's address the code. Simply stated, you want a function that will loop until the user enters a legal quantity.
product doesn't do much for the function; check it in the calling program, not here. Let the function have a single purpose: fetch a valid quantity.
Let's work from there in the standard recipe for "loop until good input". Very simply, it looks like:
Get first input
Until input is valid
... print warning message and get a new value.
In code, it looks like this.
def get_quantity():
quantity_str = input("Please enter the amount of this item you would like to purchase: ")
while not quantity_str.isdigit():
print("We didn't recognise that number. Please try again.")
quantity_str = input("Please enter the amount of this item you would like to purchase: ")
return quantity
As for coding practice ...
Develop incrementally: write a few lines of code to add one feature to what you have. Debug that. Get it working before you add more.
Learn your language features. In the code you've posted, you misuse for, in, return, and a function call.
Look up how to solve simple problems. try/except is a more difficult concept to handle than the simple isdigit.
You should try this..
def quantityFunction(product):
valid = False
while True:
if product is not None:
quantity = raw_input("Please enter the amount of this item you would like to purchase: ")
if quantity.isdigit():
return int(quantity)
valid = True
else:
print("We didn't recognise that number. Please try again.")
continue
return False
quantity = quantityFunction("myproduct")
I made a program in python that is supposed to accept a name as user input. It will then check if the name given is contained inside a string that is already given and if it is then the program will print out the telephone next to that name. My code is as follows:
tilefwnikos_katalogos = "Christoforos 99111111: Eirini 99556677: Costas 99222222: George 99333333: Panayiotis 99444444: Katerina 96543217"
check=str(input("Give a name: "))
for check in tilefwnikos_katalogos:
if check=="Christoforos":
arxi=check.find("Christoforos")
elif check=="Eirini":
arxi=check.find("Eirini")
elif check=="Costas":
arxi=check.find("Costas")
elif check=="George":
arxi=check.find("George")
elif check=="Panayiotis":
arxi=check.find("Panayiotis")
elif check=="Katerina":
arxi=check.find("Katerina")
s=check.find(" ",arxi)
arxi=s
y=check.find(":",arxi)
telos=y
apotelesma=tilefwnikos_katalogos[arxi+1:telos]
print(apotelesma)
But when I try to run it, I input the name and then the following message pops up:
Traceback (most recent call last):
File "C:\Users\Sotiris\Desktop\test.py", line 16, in <module> s=check.find(" ",arxi)
NameError: name 'arxi' is not defined
What am I doing wrong?
You're getting your error because arxi isn't getting defined in the first place when then name the user gave is not present on your list.You can fix that by simply adding an unconditional else case to your if/else if bundle as pointed in the comments. But the very way you tackled this problem is faulty, storing data like this in a string is a bad idea, you want to use a dictionary:
phone_catalog = {'Christoforos': 99111111, 'Eirini': 99556677, 'Costas': 99222222, 'George':99333333, 'Panayiotis':99444444, 'Katerina': 96543217}
Also check isn't a very clear variable name, maybe you should try using something better like:
user_name = str(input("Give a name: "))
And now you can do your if/elif condition but replacing it for using dictionary logic and making sure you have a final else, like such:
if user_name in phone_catalog:
print(phone_catalog[user_name])
else:
print("Unknown user")
See how the dictionary made your life much easier and your code cleaner here? Read more on Python Data Structures.
so there are a few things you have overlooked / not going as expected, the first of which is how iterating over strings in python works:
tilefwnikos_katalogos = "Christoforos 99111111: Eirini 99556677: Costas 99222222: George 99333333: Panayiotis 99444444: Katerina 96543217"
for check in tilefwnikos_katalogos:
print(check)
#print(repr(check)) #this shows it as you would write it in code ('HI' instead of just HI)
so check can never be equal to any of the things you are checking it against, and without an else statement the variable arxi is never defined. I'm assuming you meant to use the check from the user input instead of the one in the loop but I'm not sure you need the loop at all:
tilefwnikos_katalogos = "Christoforos 99111111: Eirini 99556677: Costas 99222222: George 99333333: Panayiotis 99444444: Katerina 96543217"
check=str(input("Give a name: ")) #the str() isn't really necessary, it is already a str.
if check=="Christoforos":
arxi=check.find("Christoforos")
elif check=="Eirini":
arxi=check.find("Eirini")
elif check=="Costas":
arxi=check.find("Costas")
elif check=="George":
arxi=check.find("George")
elif check=="Panayiotis":
arxi=check.find("Panayiotis")
elif check=="Katerina":
arxi=check.find("Katerina")
else: raise NotImplementedError("need a case where input is invalid")
s=check.find(" ",arxi)
arxi=s
y=check.find(":",arxi)
telos=y
apotelesma=tilefwnikos_katalogos[arxi+1:telos]
print(apotelesma)
but you could also just see if check is a substring of tilefwnikos_katalogos and deal with other conditions:
if check.isalpha() and check in tilefwnikos_katalogos:
# ^ ^ see if check is within the string
# ^ make sure the input is all letters, don't want to accept number as input
arxi=check.find(check)
else:
raise NotImplementedError("need a case where input is invalid")
although this would make an input of C and t give Cristoforos' number since it retrieves the first occurrence of the letter. An alternative approach which includes the loop (but not calling the variable check!) would be to split up the string into a list:
tilefwnikos_katalogos = "..."
check = input(...)
for entry in tilefwnikos_katalogos.split(":"):
name, number = entry.strip().split(" ")
if check == name:
apotelesma=number
break
else:
raise NotImplementedError("need a case where input is invalid")
although if you are going to parse the string anyway and you may use the data more then once it would be even better to pack the data into a dict like #BernardMeurer suggested:
data = {}
for entry in tilefwnikos_katalogos.split(":"):
name, number = entry.strip().split(" ")
data[name] = number #maybe use int(number)?
if check in data:
apotelesma = data[check]
else:
raise NotImplementedError("need a case where input is invalid")
I am very new to Python (started 2 days ago). I was trying to validate positive integers. The code does validate the numbers but it asks twice after a wrong input is entered. For example if I enter the word Python, it says: This is not an integer! like is supposed to but if I enter 20 afterwards, it also says it is not an integer and if I enter 20 again it reads it.
def is_positive_integer(input):
#error: when a non-integer is input and then an integer is input it takes two tries to read the integer
flag = 0
while flag != 1:
try:
input = int(input)
if input <= 0:
print "This is not a positive integer!"
input = raw_input("Enter the number again:")
except ValueError:
print "This is not an integer!"
input = raw_input("Enter the number again: ")
if isinstance(input, int):
flag = 1
return input
number = raw_input("Enter the number to be expanded: ")
is_positive_integer(number)
number = int(is_positive_integer(number))
Any help is appreciated.
The main bug is that you call is_positive_integer(number) twice with the same input (the first thing you enter).
The first time you call is_positive_integer(number), you throw away the return value. Only the second time do you assign the result to number.
You can "fix" your program by removing the line with just is_positive_integer(number) on its own.
However, your code is a little messy, and the name is_positive_integer does not describe what the function actually does.
I would refactor a little like this:
def input_positive_integer(prompt):
input = raw_input(prompt)
while True:
try:
input = int(input)
if input <= 0:
print "This is not a positive integer!"
else:
return input
except ValueError:
print "This is not an integer!"
input = raw_input("Enter the number again: ")
number = input_positive_integer("Enter the number to be expanded: ")
The problem stems from the fact that you're calling is_positive_integer twice. So, the first time it's called, you send it a string like 'hello', then it says it's not an integer and tells you to try again. Then you enter '20', which parses fine, and it's returned.
But then you don't save a reference to that, so it goes nowhere.
Then you call the function again, this time saving a reference to it, and it first tries the original bad string, which was still there in number. Then it complains that it's a bad input, asks you for a new one, and you provide it, terminating the program.
I spent a good hour or more looking for the answer on here. I have found a few things that help, but do not answer my question specifically. I am using Python 3.3.3. I am a novice so please be gentle.
I am trying to create a program that takes a user input, but then I need to do a check to see what datatype that input is, and then based on that datatype take a certain course of action.
Any string besides those found in this list:
valid_help_string_list = ['\'help\'', '\'HELP\'', 'help', 'HELP']
should result in the printing of:
'please enter a valid entry' or something to that effect.
Any integer (over 0 but under 500) should have float() used on it to make the rows line up.
Any float (over 0.0 but under 500.0) is valid.
For the sake of this project I am assuming nobody using this will weigh under 100 lbs or over 500.
Anything not falling within those categories should also yield the same "please enter a valid response" error message to the user.
I think it's simple enough of a project to take on for a novice. The program is meant to allow you to input your weight and then creates a pictogram based on that weight and saves it all on the next open line of the .txt file I have set up for it. Or if you want to see the legend for the pictogram, you should be able to type help in any variation found in that list.
Any help would be much appreciated.
The user input will be a string by default, so we need to check whether it could become an integer or float. As you want to turn the integers in floats anyway, there's no need to do anything complex:
def validate_input(val, min_v=100, max_v=500):
try:
val = float(val)
except ValueError:
print("Not a valid entry")
else:
if not min_v < val <= max_v:
print("Value should be between {} and {}".format(min_v, max_v))
else:
return val
return False
Now your calling loop can read:
while True:
val = input("...")
if val in valid_help_string_list:
# print help
else:
val = validate_input(val)
if val:
break
# use val
Note that this relies on the return from validate_input being either False or a number larger than 0; Python will interpret a zero return as False and not reach the break, so I recommend keeping min_v >= 0.