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removing useless stuff and adding useful stuff
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‎files.md‎

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@@ -69,7 +69,9 @@ Let's create a file and write a hello world to it.
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Doesn't seem like it did anything. But actually it created a `hello.txt`
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somewhere on our system. On Windows it's probably in `C:\Users\YourName`,
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and on most other systems it should be in `/home/yourname`. You can open
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it with notepad or any other plain text editor your system comes with.
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it with notepad or any other plain text editor your system comes with by
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opening the folder that contains the file and then double-clicking the
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file.
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So how does that code work?
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@@ -82,23 +84,22 @@ object that is assigned to the variable `f`.
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>>>
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```
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So the first argument we passed to `open` was the path we wanted to write.
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File objects are not the same thing as paths and filenames, so if we try
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to use `'hello.txt'` like we used `f` it doesn't work.
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The first argument we passed to `open` was the path we wanted to write.
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Our path was more like a filename than a path, so the file ended up in
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the current working directory.
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The second argument was `w`... but where the heck does that come from?
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`w` is short for write, and that just means that we'll create a new file.
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There's some other modes we can use also:
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The second argument was `w`. It's short for write, and that just means
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that we'll create a new file. There's some other modes we can use also:
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| Mode | Short for | Meaning |
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|-------|-----------|-----------------------------------------------------------------------|
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| `r` | read | Read from an existing file. |
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| `w` | write | Write to a file. **If the file exists, its old content is removed.** |
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| `a` | append | Write to the end of a file, and keep the old content. |
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The `w` and `a` modes create a new file if it exists already, but trying
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to read from a non-existent file is an error.
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But what is that `with ourfile as f` crap? That's just a fancy way to make
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sure that the file gets closed, no matter what happens. As we can see,
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the file was indeed closed.
@@ -130,9 +131,9 @@ we created to a list of lines.
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>>>
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```
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Trying to open a non-existent file with `w` created the file for us, but
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doing that with `r` gives us an error instead. We'll learn more about
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errors [later](exceptions.md).
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Trying to open a non-existent file with `w` or `a` creates the file for
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us, but doing that with `r` gives us an error instead. We'll learn more
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about errors [later](exceptions.md).
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```py
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>>> with open('this-doesnt-exist.txt', 'r') as f:
@@ -150,11 +151,11 @@ that `\n` doing there?
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`\n` means newline. Note that it needs to be a backslash, so `/n`
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doesn't have any special meaning like `\n` has. When we wrote the file
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with print it actually added a `\n` to the end of it. It's good practise
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with print it actually added a `\n` to the end of it. It's recommended
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to end the content of files with a newline character, but it's not
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necessary.
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So how does that work if we have more than one line in the file?
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Let's see how that works if we have more than one line in the file.
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```py
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>>> with open('hello.txt', 'w') as f:
@@ -206,7 +207,8 @@ that reads the next line, and returns `''` if we're at the end of the file.
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```
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There's only one confusing thing about reading files. If we try
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to read it twice we'll find out that it only gets read once:
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to read the same file object twice we'll find out that it only gets read
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once:
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```py
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>>> first = []
@@ -226,8 +228,8 @@ to read it twice we'll find out that it only gets read once:
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File objects remember their position. When we tried to read the
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file again it was already at the end, and there was nothing left
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to read. But if we open the file again, it's in the beginning
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again and everything works.
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to read. But if we open the file again, we get a new file object that
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is in the beginning and everything works.
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```py
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>>> first = []
@@ -250,21 +252,7 @@ again and everything works.
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Usually it's best to just read the file once, and use the
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content we have read from it multiple times.
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As we can see, files behave a lot like lists. [The join string
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method](https://docs.python.org/3/library/stdtypes.html#str.join) joins
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together strings from a list, but we can also use it to join together
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lines of a file:
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```py
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>>> with open('hello.txt', 'r') as f:
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... full_content = ''.join(f)
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...
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>>> full_content
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'Hello one!\nHello two!\nHello three!\n'
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>>>
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```
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But if we need all of the content as a string, we can just use [the read
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If we need all of the content as a string, we can use [the read
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method](https://docs.python.org/3/library/io.html#io.TextIOBase.read).
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```py
@@ -297,8 +285,8 @@ adding an `r` to the beginning of the string. In this case the `r`
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is short for "raw", not "read".
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```py
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>>> r'C:\some\name' == 'C:\\some\\name'
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True
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>>> r'C:\some\name'
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'C:\\some\\name'
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>>>
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```
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@@ -312,6 +300,11 @@ don't contain backslashes we don't need to double anything or use
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>>>
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```
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Doing things like `open('C:\\Users\\me\\myfile.txt', 'r')` is not
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recommended because the code needs to be modified if someone wants to
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run the program on a different computer that doesn't have a
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`C:\Users\me` folder.
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## Examples
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This program prints the contents of files:

‎loops.md‎

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@@ -297,6 +297,114 @@ of `stuff.copy()` and `stuff[:] = []` instead of `stuff.clear()`.
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- The `break` keyword can be used to interrupt the innermost loop at
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any time.
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## Examples
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Repeat something an endless amount of times.
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```py
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message = input("What do you want me to say? ")
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while True:
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print(message, "!!!")
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```
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Ask the user to enter five things and print them.
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```py
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things = []
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print("Enter 5 things: ")
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while len(things) < 5:
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thing = input("> ")
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things.append(thing)
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print("You entered these things:")
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for thing in things:
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print(thing)
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```
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Ask the user a bunch of questions.
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```py
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questions_and_answers = [
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# [question, answer], ...
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["What is 2+4? ", "6"],
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["What is 2-4? ", "-2"],
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["What is 2*4? ", "8"],
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["What is 2/4? ", "0.5"],
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# You could add more questions, but the code that asks them
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# wouldn't need to be changed in any way.
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]
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for qa in questions_and_answers:
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while True:
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if input(qa[0]) == qa[1]:
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print("Correct!")
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break
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else:
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print("That's not what I was thinking of... Try again.")
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```
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Store a list of names and let the user check if the program knows
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the user.
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```py
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# You can add names here so the program will know them automatically
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# when it starts.
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namelist = []
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print("Options:")
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print(" 0 Quit")
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print(" 1 Check if I know you")
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print(" 2 Introduce yourself to me")
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print(" 3 Make me forget you")
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print(" 4 Print a list of people I know")
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print() # print an empty line
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while True:
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option = input("Choose an option: ")
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# Things like option == 0 don't work because option is a string
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# and it needs to be compared with a string.
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if option == '0':
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print("Bye!")
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break
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elif option == '1':
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name = input("Enter your name: ")
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if name in namelist:
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print("I know you! :D")
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else:
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print("I don't know you :/")
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elif option == '2':
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name = input("Enter your name: ")
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if name in namelist:
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print("I knew you already.")
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else:
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namelist.append(name)
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print("Now I know you!")
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elif option == '3':
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name = input("Enter your name: ")
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if name in namelist:
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namelist.remove(name)
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print("Now I don't know you.")
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else:
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print("I didn't know you to begin with.")
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elif option == '4':
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if len(namelist) == 0: # len is short for length
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print("I don't know anybody yet.")
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else:
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for name in namelist:
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print("I know %s!" % name)
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else:
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print("I don't understand :(")
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print()
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```
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## Exercises
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1. Back in "Using if, else and elif" we created a program that asked

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