# File reading and writing in python
:::info
This is a note part of the coding course for IB Computer Science program. If you want to check out other parts of the same course you can see them in [This link of teaching material](/68GDv_RgT-yh9oERMvdnFw)
:::
:::warning
:warning: Note still under construction :warning:
:::
## Context
In this new syllabus you're suposed to know how to handle files, reading and writing them.
### What is a file. File systems, files and file extensions
:::info
This is the link to [A1.1](/6LgCmT4IToOkOu0CuV3UBQ) and [A1.3 Notes and Resources](/iD8iagfzSm6tbNqeApW50g)!
:::
Just to be in one page remember that the OS is the one that is going to convert this
Into this:
_Linux_
or this
_windows_
This is done through _abstraction_. These folders can represent parts of the secondary memory and there can be loops or shorcuts to other places.
To know where your stuff is you will need to know the _route_
For example (windows)
A file that you download in here an be
C:\Users\your.name\Downloads\ib_guide.pdf
C:\ is the **disk** (to be precise the partition that has been *mounted* so it can be accessed)
Users, your.name and Downloads are **folders**
ib_guide.pdf is the name of the file and it consists of two parts:
ib_guide is the name
and .pdf (including the dot) is the **extension** and it tells the OS how it should be read that file.
:::info
You can change the extension of a file and it will change how the OS is going to interpret it but if you change a .docx file to a .jpg don't expect to be converted actually into a jpg (or a pdf).
:::
### Types of file formats
//TO-DO
### Absolute routes and relative routes
Programs also can access files. Usually they can be linked through **absolute routes** that will start from Home or from the disk unit or **relative routes** that will start from the folder where it's being executed the program.
For example, `C:\Users\your.name\Downloads\ib_guide.pdf` is an absolute route, but `\Downloads\ib_guide.pdf` is a relative route from the folder C:\Users\your.name
:::warning
:warning: **Slashes**: In windows the slashes are these ones: `\`, in macOS and Linux is the other slash: `/`. In URLS it's also the `/`
:::
Relative routes are usually more practical because it can be transfered from one device to another.
The most simple **relative route** is the same folder where the script is being executed. That's only the name of the file.
## Opening files in python
:::info
This is complicated be used with the one compiler for _reasons_
:::
We should use the method open and put the return of that function into a variable (usually `file` but can be other)
```python!
file = open("testinput.txt")
print(file)
```
Now, this file is an Object so when we try to print it this is going to be the result:
output:
If we want to actually read the content we need to use the function `read()` that, if we don't specify is going to read line by line.
For that we use a loop for, commonly just:
```python!
file = open("testinput.txt")
for line in file:
pass #substitute with whatever you're going to do with the line
```
### Opening modes
We have different modes
r-> read
w -> write
a -> append
r+ -> read and write.
If we don't give the input it will be read by default. If we want to change the way we do it, we can use a string as a second parameter.
:::danger
:warning: Writing is not the same as appending. If you write a file you're overriding what it was before.
:::
What if I want to "edit" a file? Usually you read it, you store in a variable and then you write with the appropiate modifications.
```python!
file = open("testinput.txt", "a")
print(file)
```
```python!
file = open("testinput.txt","a")
file.writelines("\nWe're the crystal gems")
file.close()
```
For making a new line we need to use special characters. The new line is usually `\n`.
:::info
**Special characters in Python**
These special characters are common in other programming languages
_[(source)](https://www.w3schools.com/python/gloss_python_escape_characters.asp)_
:::
### Closing files: close()
Remember from 1.3. that Operating Systems are in charge to allocate the access to files and folders?
So it happens that if Python (or other program) opens a file, the Operating System is going to prevent other programs to access to the file itself. This is something that you may have encounter when trying to delete a file that you have opened.
So for making those files free for other programs (or the user) to read/write, we need to close the files once we have finished with them. As you might expect this is done with the `close()` function.
```python!
file = open("testinput.txt")
file.close()
```
If you don't write it, once the program is over the interpreter is going to close the file anyway.
But writing this is what is called a **good practice**. Usually the outputs are going to be the same, but other more subtle benefits can happen from this. In this case, that if you do a more complex program you're not going to block files.
### With open
Regarding that we have the problem that we have in the while, that we may not remember when to close the file. For that in python there is other common structure that is with open.
The idea is that you need to write "with open(parameters) as nameOfTheFileVariable:"
Whatever happens inside that block will end with a `close()`, so you don't need to write it!
So if I write this:
```python!
file = open("myFile.txt")
for line in file:
print(line)
file.close()
```
It's the same as writing this:
```python!
with open("myFile.txt") as file:
for line in file:
print(line)
```
You will see examples with both versions all over the internet.
## Example of reading
Imagine that we have this textFile.txt
```
Pipo
Pepa
Pepe
Thomas Jefferson
```
And we want to create a list on them.
One solution is create in our `main.py` the following code:
```python=
file = open("textFile.txt")
myList = list()
for line in file:
myList.append(line)
print(myList)
```
In this script we open the file, we create an empty list and then for each line that there is in the file we append it to the list. Finally we print the result. And this is the result:
HUM!
Then we need to use strip() to take that \n.
:::info
`\n` is an escape caracter that means to be a new line.
:::
:::success
You can read more about strip here: https://hackmd.io/y_afcRmLQcysv05xlSqRuQ#strip
:::
```python
file = open("textFile.txt")
myList = list()
for line in file:
myList.append(line.strip())
print(myList)
```
### Common error: encoding
Remember when in [A1.2 Binary representation](/hQH2iVecTpOaEu WNjLqR3Q) that we talk about ASCII and unicode with UTF-8 and UTF-32? It happens that if you open a file that it's written in other encoding, special characters (such as "ñ", cirilic characters or chinese characters) are going to be seen _weird_. To solve it we need to inlcude one optional parameter and just say `encoding='utf-8'`
It's the same as writing this:
```python!
with open("myFile.txt",encoding='utf-8') as file:
for line in file:
print(line)
```
## How to organize using OneCompiler several files so we can open other files as data files
Open the one compiler and work on the script.
Press the "+" and new python file
**RENAME** the file name and extension
(in this case textFile.txt but we can change the name or the extension)
Change the content:
That's it! you can work with this tests.
:::warning
This doesn't work for writing files
:::
## Example of writing a file
Now let's say that we want to do the other way around:
I want to have the same file that it's the example in the writing part:
We can start with the list that we have before and we need to prepare the lines with the special character `\n`
So let's start here:
```python!
myList = ["Pipo", "Pepa", "Pepe", "Thomas Jefferson"]
```
Now we need to open the file:
```python!
myList = ["Pipo", "Pepa", "Pepe", "Thomas Jefferson"]
with open("output.txt", "w" ,encoding='utf-8') as file:
#Don't forget the w of writing!
pass #remember that this is going to be changed later
```
And while the file is open we can create the lines and write them:
```python!
myList = ["Pipo", "Pepa", "Pepe", "Thomas Jefferson"]
with open("output.txt", "w" ,encoding='utf-8') as file:
for element in myList:
file.write(element)
file.write("\n")
```
If done correctly this will do this:
Problem: we have 5 lines in the file but we only want to have 4.
//TO-DO by the students.
## Exercises
### Write execises with specific propertis using range
range() is a function than we can use to create fast lists so we can create easily files with specific number
```python!
myList = list(range(1,11)) #numbers from 1 to 10
with open ("testfile.txt", "w") as file:
for index in range(len(myList)):
file.write(str(myList[index])) #we need to convert the integers to strings
if index < len(myList)-1:
file.write("\n")
```
With that structure you can do:
Construct in Python a program that writes a file called "testOutput.txt" with all the numbers from 1 to 100
//TO-DO by the students
Construct in Python a program that writes a file "testOutput.txt" with all the numbers from 100 to 1000
//TO-DO by the students
Construct in Python a program that writes a file "testOutput.txt" with all the numbers from -4 to -10
//TO-DO by the students
Construct in Python a program that writes a file "testOutput.txt" with all the **even** numbers from 100 to 1000
Implementation 1:
```python!
file = open("testOutput.txt", "w")
for x in range(100,1001,2):
string = str(x)
file.write(string)
file.write("\n")
file.close()
```
Implementation 2 (if you don't remember steps on range)
```python!
# even numbers from 100 to 1000
file = open("testOutput.txt", "w")
for x in range(100,1001):
if(x%2==0):
string = str(x)
file.write(string)
file.write("\n")
file.close()
```
Construct in Python a program that writes a file "testOutput.txt" with all the numbers with 3 digits that the 3 digits sum 9 (such as 117, 333) being the smaller one the first.
//TO-DO by the students
### Replacing quote of Shakespeare
First version:
```python!
# to be or not to be by Shakespeare
quote = "To be, or not to be, that is the question: Whether 'tis nobler in the mind to suffer The slings and arrows of outrageous fortune, Or to take arms against a sea of troubles And by opposing end them. To die—to sleep"
file = open("shakespeareQuote.txt", "w")
vowels = ["a", "e", "i", "o", "u"]
quoteA = quote
for vowel in vowels:
quoteA = quoteA.replace(vowel, "a")
file.write(quoteA)
file.write("\n")
```
Second version
```python!
# to be or not to be by Shakespeare
quote = "To be, or not to be, that is the question: Whether 'tis nobler in the mind to suffer The slings and arrows of outrageous fortune, Or to take arms against a sea of troubles And by opposing end them. To die—to sleep"
file = open("shakespeareQuote.txt", "w")
vowels = ["a", "e", "i", "o", "u"]
for vowelSubstituting in vowels:
quoteOutput = quote
for vowel in vowels:
quoteOutput = quoteOutput.replace(vowel, vowelSubstituting)
file.write(quoteOutput)
file.write("\n")
```
### Problem of using indexes
I usually advise that indexes are cool to use in list because sometime __you need to__. But with files we can't. If I write this script
```python!
file = open("myText.txt")
print(file)
sum = 0
for lineIndex in range(len(file)):
pass
```
The result would be this
```python!
file = open("myText.txt")
print(file)
lineIndex = 0
for line in file:
print(lineIndex, line)
if lineIndex == 2:
print("This is the one that we need")
break
#doSomething
lineIndex +=1 #update Index
```
### Exam like exercise Advent of code
For this we're going to use this example of the Advent Of Code of 2024 https://adventofcode.com/2024/day/1
The actual thing is a bit more complicated so let's start easy. Then we can continue with the different versions.
This is the file that we need to read:
```
4 3
2 5
1 3
3 9
3 3
```
You can paste it into some input.txt or similar.
If you want, you can test the actual file of the input that is a long file and I'm writing after this spoiler:
:::spoiler
```
56208 95668
52621 74203
95252 33335
79799 26047
88005 37435
61887 93836
48454 95821
62543 40154
68631 73255
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36649 79056
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18519 64707
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47018 75935
30452 48882
12559 58817
32825 91051
16051 19974
80171 17022
19374 54710
57295 84738
81314 38097
33479 49965
38611 79056
63785 49754
18774 70415
98510 72330
85801 73475
10744 17022
15097 11280
52514 37435
86860 96099
27804 22005
62601 79155
86020 70452
26991 15399
98690 69989
18491 79768
85009 79056
11329 20212
10782 70405
85026 40727
26888 34001
36315 42445
49098 37435
59474 38097
86676 41947
55178 84321
53985 87526
65575 47299
57292 82817
24657 37081
82204 10439
82639 73475
81107 74139
89946 97271
93306 41394
62863 46550
48547 86956
12374 55301
81681 95539
94195 84321
15303 24017
36395 93928
28572 10804
62931 45406
33902 88845
25697 83483
86956 51476
73957 55123
83525 29803
16581 19887
96012 90129
14067 40727
66384 38351
43782 75373
53517 29489
17126 71797
10854 68046
68594 89990
89786 17022
81737 11280
71138 84321
77457 62041
99540 54740
15369 87693
73475 39716
35348 99949
67944 97271
91345 20757
48703 99901
56450 84321
75169 70680
27015 45299
34175 85030
21184 64377
95716 97271
25738 70254
79049 75062
54904 68773
12813 47161
95857 74505
17456 87526
24008 30203
92925 17022
17008 98227
53399 13325
84080 49498
23522 62070
80392 37081
67497 89518
31797 46550
67380 29610
29237 47817
32106 13477
17711 61786
19802 35480
46456 37081
17560 67389
59598 64837
30808 10592
78213 86193
34411 89045
22714 61786
88368 79584
49172 37435
28106 30525
34595 65269
67363 90595
50389 33649
57932 74115
85665 15566
68846 84257
37420 75483
70969 85030
53989 74115
30509 81520
92232 63922
12314 64612
59372 76983
31019 75373
28072 58019
51728 67527
82597 36949
66512 46706
89805 89043
13220 72753
19121 31313
27852 89644
29411 97271
10756 88672
60773 82080
81746 11688
90653 23550
59400 14905
97489 44079
20132 40678
82210 43722
90436 80985
92889 18182
40727 19573
76922 17022
27554 78420
47592 87324
78580 75728
91390 63922
82409 80324
60005 53958
46353 90334
89672 26377
52436 11280
71495 41947
59787 93836
13621 79584
16093 31156
11603 51436
31593 12955
70829 89990
61100 96353
71072 61786
33450 17022
65493 48359
51250 44884
49691 61786
41947 61786
60973 93836
44926 87954
90989 35723
98883 30386
83974 67713
86576 17323
32534 77745
57475 32708
86499 43190
26325 71687
16737 58398
88460 17022
10798 61963
59899 54790
84722 56390
38117 75766
12595 97271
23267 78757
70504 97271
44104 63922
22950 85030
66137 84321
18407 99949
37598 61786
28201 24323
19404 79868
53725 33781
56941 41947
88231 83925
30916 73475
81019 46011
91386 23123
29455 79332
32603 42378
61578 46922
67431 12955
85884 15692
40799 89045
27326 97271
79681 11280
35614 38097
99407 54190
30288 10124
59805 66109
27031 12955
51952 46550
63420 17022
96282 17307
45123 17022
95858 32553
85339 98702
19887 81650
20177 66465
92548 84663
28915 38097
48995 58398
32153 97259
14697 55843
15115 31921
97258 21175
11833 75150
20476 37081
57780 87869
20868 16813
55154 72786
51855 79056
58551 94071
32367 60493
70926 16754
95196 94004
30949 32933
93955 10128
14553 61786
67116 11579
34377 85680
69983 79584
92220 38097
69116 11280
83291 10124
63499 30127
48167 97271
70014 13990
40226 78226
71699 12411
40455 75750
74381 11343
88846 74967
17219 79739
78747 30775
98201 18354
54571 68841
85738 49837
17375 10897
44862 46550
15932 11030
79584 17022
77641 58398
54882 38097
87246 75159
33635 41878
75249 60331
46712 55151
48228 91967
17264 39458
58907 86956
62693 87904
72122 43222
70553 73934
43027 90334
69420 97905
91527 10124
34146 96995
77998 88728
80279 91008
35125 77608
33786 63922
82027 38917
10855 49903
66407 38097
50926 49360
73591 97271
97759 84321
82742 49137
16002 57753
53455 38875
66703 40727
76244 92842
87526 49664
92147 87526
```
:::
First, we want to go for line by line. Since the element that are separating the numbers
```python!
file = open("input.txt")
for line in file:
print(line.split())
```
If we do this with the first line of the extended file we're going to get this:
`['56208', '95668']`
See the quotes? Put a pin on those.
Now let's supose that we want to add each of these 2 numbers. If we do this:
```python!
file = open("input.txt")
for line in file:
numbers = line.split()
total = numbers[0] + numbers [1]
print(total)
```
We're going to get this:
`5620895668` for the first line
This is an error. Do you know why? Give it a thought.
:::spoiler
56208 + 95668 cannot be a number that big as 5620895668
:::
Why? Because `'56208'` (and `'95668'`) are strings, so if you put a + between them, you're going to **concatenate them**, so if we want to treat them as numbers we need to treat them as numbers (that is another type). For that we need to use **casting**
#### Casting
_Mold and cast of a fossil_
Casting is treat one type of data as another type of data. Usually to cast we use the _constructor_ of the type that we want. For example if we want the "5432" string to be taken as a number, we write `int("5432"`
```python!
file = open("input.txt")
for line in file:
numbers = line.split()
total = int(numbers[0]) + int(numbers[1])
print(total)
```
#### Sum the values of the lines
//TO-DO
#### Sum the values of the columns
//TO-DO
## Exam exercises
### Maximum number in a file
There is a file called numbers.txt with 1000 lines of numbers. In the next figure you can see a fraction of this numbers:
Figure: extract from the file.
Construct in Python an algorithm that outputs the highest number of this list. [4]
Solution:
:::spoiler
Option 1:
Using `max()`
```python!
file = open("filename.txt")
numbers = list()
for line in file:
numbers.append(int(line))
print(max(numbers))
```
Option 2:
Not using `max()`
```python!
file = open("filename.txt")
numbers = list()
for line in file:
numbers.append(int(line))
maxNumber = numbers[0]
for number in numbers:
if number>maxNumber:
maxNumber = number
print(maxNumber)
```
:::
### Minimum number in a file
There is a file called numbers.txt with 1000 lines of numbers. In the next figure you can see a fraction of this numbers:
Figure: extract from the file.
Construct in Python an algorithm that outputs the lowest number of this list. [4]
### Write a file with a range of numbers
Construct in Python a program that writes a file called “output.txt” with the numbers that comes from 3000 to 1000 in descending order, each number in a line [5]
Variation:
Construct in Python a program that writes a file called “output.txt” with the numbers that comes from 1000 to 1 in descending order, each number in a line [5]
### Exam exercise the CSV of countries
:::info
:link: This exercise combines 2D lists that you can look for it here https://hackmd.io/-ylq-DUoTKOS6qGCiq3JRA
:::
I have a CSV downloaded from the Canarian Statistics Institute (From the Canary Islands, Spain) with the names of the countries and their coordinates. The full project needs to create a map. The file has this structure (the grey text is the line number, it’s not part of the file but the number that is nonconsecutive is part of the line)
The information required is the name of the country and its coordinates (longitude and latitude).
A- Construct in Python an algorithm that reads “data.csv” and creates a 2D list called countries with the following structure: [5]
:::info
This is way easier splitting, to check a similar exercise focus in the split part you can go here: https://hackmd.io/y_afcRmLQcysv05xlSqRuQ#Process-data-from-a-line-in-a-csv-file
:::
Solution:
:::spoiler
Implementation 1:
```python!
file = open("data.csv") #one mark with the correct opening
countries = []
for line in file: #one mark with the correct loop
countries.append([])
fields = line.split(",") #1 mark for the split
countries[-1].append(fields[1]) #append the second element
countries[-1].append(float(fields[3])) #if you miss the casting done with float it's ok
countries[-1].append(float(fields[4])) #if you miss the casting done with float it's ok
```
Implementation 2:
```python!
file = open("data.csv") #one mark with the correct opening
countries = []
row = 0
for line in file: #one mark with the correct loop
countries.append([])
fields = line.split(",") #1 mark for the split
countries[row].append(fields[1]) #append the second element
countries[row].append(float(fields[3])) #if you miss the casting done with float it's ok
countries[row].append(float(fields[4])) #if you miss the casting done with float it's ok
row = row +1
```
Implementation 3 (forgot how to split):
```python!
file = open("data.csv") #one mark with the correct opening
countries = []
for line in file: #one mark with the correct loop
countries.append([])
fieldIndex = 0
currentField = ""
for char in line:
if char != "," and char != "\n": #\n is the special character for "end of the line" I could give the mark if forgot
currentField = currentField + char
else:
if fieldIndex == 1 or
fieldIndex == 3 or
fieldIndex == 4:
countries[fieldIndex] = currentField
fieldIndex +=1
fieldIndex = fieldIndex +1
currentField = ""
```
Another implementation without split and "\n" using index while looping through the line.
```python!
file = open("data.csv") #one mark with the correct opening
countries = []
for line in file: #one mark with the correct loop
countries.append([])
fieldIndex = 0
currentField = ""
for lineIndex in range(len(line)): #lineIndex can be named li or something shorter in an exam
if line[lineIndex] != "," and lineIndex != len(line)-1:
currentField = currentField + char
else:
if fieldIndex == 1 or
fieldIndex == 3 or
fieldIndex == 4:
countries[fieldIndex] = currentField
#this implementation doesn't cast the numbers to float
fieldIndex +=1
fieldIndex = fieldIndex +1
currentField = ""
```
Another inplementation with no split:
```python!
file = open("data.csv") #one mark with the correct opening
countries = []
for line in file: #one mark with the correct loop
countries.append([
line[line.find(",")+1:line.find(",PAISES")],
line[line.find(",PAISES,")+8:line.rfind(",")],
line[line.rfind(","):]
])
```
:::
This exercise continues validating the 2D list here https://hackmd.io/RCQ32fpjS-up6rYC74jPoQ#Validating-country-data
## Reference
[W3Schools](https://www.w3schools.com/python/python_file_handling.asp)
## Extra: copying files for a printer
As far as I know IB doesn't ask you to use the library `shutil` (that I guess that is "shell utility" being shell the terminal) here is a fancy code for something that I needed to do and automate the boring stuff.
I want to print some cards in a printer and they print them in decks and their decks have 54 cards (like poker decks). So I need to send the files separately. Usually I will vary the side a (Anverso in Spanish) and the side b (Reverso in Spanish). Here we're going to use an example where I want to print both sides the same.
:::warning
Important, I'm using here a **relative route** to write, but the folder **needs to exist** or you will get an error of "No such file or directory"
:::
```python=
import shutil
numberOfCopies = 54
sourcePathSideA = "Anverso.pdf"
sourcePathSideB = "Reverso.pdf"
for x in range(numberOfCopies):
#This if is so I create Anverso 01 instead of Anverso 1 and have the same number of characters
if x <9:
stringNumber = "0" + str(x+1)
else:
stringNumber = str(x+1)
copySideARoute= "Anversos/Anverso "+ stringNumber + ".pdf"
copySideBRoute= "Reversos/Reverso "+ stringNumber + ".pdf"
shutil.copy(sourcePathSideA, copySideARoute)
shutil.copy(sourcePathSideB, copySideBRoute)
```
And with less of 20 lines of code we're done!
::: info
I don't use the file open and file read because pdf has some particularities with encoding (they can have different encodings inside it) so using this tool is easier. But here you have the code that should work with plain text files:
:::danger
Not working code
```python
numberOfCopies = 54
sideAOriginal = open("Anverso.pdf", encoding= "utf-8") #sideA
print(sideAOriginal)
sideBOriginal = open("Reverso.pdf", encoding= "utf-8") #sideB
for x in range(numberOfCopies):
#This if is so I create Anverso 01 instead of Anverso 1 and have the same number of characters
if x <9:
stringNumber = "0" + str(x+1)
else:
stringNumber = str(x+1)
copySideARoute= "Anversos/Anverso_"+ stringNumber + ".pdf"
copySideBRoute= "Reversos/Reverso_"+ stringNumber + ".pdf"
with open(copySideARoute, "w", encoding= "utf-8") as copySideA:
copySideA.write(sideAOriginal.read())
with open(copySideBRoute, "w", encoding= "utf-8") as copySideB:
copySideB.write(sideBOriginal.read())
```
:::
:::
### Reference for the copying part
https://stackoverflow.com/questions/123198/how-do-i-copy-a-file
https://docs.python.org/3/library/shutil.html#directory-and-files-operations
## Snippets
```python!
fileData= str(list(range(1,11)))[1:-1]
with open ("testfile.txt", "w") as file:
file.writelines(fileData)
```
```python!
fileData= str(list(range(1,11)))[1:-1].replace(", ", "\n")
with open ("testfile.txt", "w") as file:
file.writelines(fileData)
file.close()
```
```python!
myList = list(range(1,11))
string = str(myList)[1:-1]
stringWithLines = string.replace(", ", "\n")
with open ("testfile.txt", "w") as file:
file.writelines(stringWithLines)
file.close()
```
```python!
#prepare the string
myList = list(range(1,10001))
string = str(myList)[1:-1]
stringWithLines = string.replace(", ", "\n")
#write the file
with open("testfile.txt", "w") as file:
file.writelines(stringWithLines)
file.close()
```
```python!
file = open("testinput.txt")
print(file)
for line in file:
print(line)
splittedLine = line.split(" ")
mappedLine = map(float, splittedLine)
print(mappedLine)
numbers = list(mappedLine)
print(numbers)
numbers = list(map(int, line.split(" ")))
```
```python!
file open(patata.txt)
start = True #Flag variable
for line in file:
if start:
start = False
continue
if (condition):
#dosomething
```
```python!
file open(patata.txt)
foundIncludedPlugins = False
plugins = []
for line in file:
if line == "Included plugins:":
foundIncludedPlugins = True
if foundIncludedPlugins and line[0] == " ":
plugins.append(line)
```
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