2 · Strings · bytes · IO · collections

🈁 Strings and bytes

---

👇 Strings

>>> '"Where is our homework?"'
'"Where is our homework?"'
>>> "this" "weekend"
'this weekend'
>>> """on
    github"""
'on\ngithub'

.encode("utf-8") 🔄 .decode("utf-8")

>>> b"\00\42\24\00"
b'\x00"\x14\x00'
>>> rb"\00\42\24\00"
b'\\00\\42\\24\\00'

☝️ Bytes

---

💬 Escape sequences, raw strings

>>> print("\tell me")
ell me
>>> print(r"\tell me")
\tell me

see all here

---

str � unicode

>>> s = "мем"
>>> ascii(s)
"'\\u043c\\u0435\\u043c'"
>>> repr(s)
"'мем'"
>>> list(s)
['м', 'е', 'м']
>>> s[0], type(s[0])
('м', <class 'str'>)
>>> list(s[0])
['м']

😮 Unicode 13.0 – 143 859 characters

---

…what are those for?

>>> ord("💅")
128133
>>> chr(128133)
'💅'
>>> chr(ord("💅"))
'💅'

---

🤔 How much bits (bytes) are used to represent symbols?

hint: encoding

---

PEP-393

Universal Character Set

N bytes for each symbol

• UCS-1 ⬅️ ASCII
• UCS-2 ⬅️ max code < 2
• UCS-4 ⬅️ everything else

>>> list(map(ord, "hello"))
[104, 101, 108, 108, 111]
>>> list(map(ord, "привет"))
[1087, 1088, 1080, 1074, 1077, 1090]
>>> list(map(ord, "🧐🐍➡️🤬💀"))
[129488, 128013, 10145, 65039, 129324, 128128]

---

💨 Quick basic methods overview

---

1️⃣ Registry modificators

>>> "foo bar".capitalize()
'Foo bar'
>>> "foo bar".title()
'Foo Bar'
>>> "foo bar".upper()
'FOO BAR'
>>> "foo bar".lower()
'foo bar'
>>> "foo bar".title().swapcase()
'fOO bAR'

---

2️⃣ Nice output

>>> "foo bar".ljust(16, '~')
'foo bar~~~~~~~~~'
>>> "foo bar".rjust(16, '~')
'~~~~~~~~~foo bar'
>>> "foo bar".center(16, '~')
'~~~~foo bar~~~~~'

---

3️⃣ Removing symbols

>>> "]>>foo bar<<[".lstrip("]>")
'foo bar<<['
>>> "]>>foo bar<<[".rstrip("[<")
']>>foo bar'
>>> "]>>foo bar<<[".strip("[]<>")
'foo bar'
>>> "\t foo bar \r\n ".strip()
'foo bar'

---

4️⃣ Splitting strings

>>> "foo,,,bar".split(",")
['foo', '', '', 'bar']
>>> "foo,bar,baz".partition(",")
('foo', ',', 'bar,baz')

---

5️⃣ Joining strings

>>> ", ".join(["foo", "bar", "baz"])
'foo, bar, baz'
>>> ", ".join(filter(None, ["", "foo"]))
'foo'
>>> ", ".join("bar")
# ???

---

6️⃣ Finding things

>>> "foo" in "foobar"
True
>>> "kek" not in "foobar"
True
>>> "foobar".startswith("foo")
True
>>> "foobar".endswith(("boo", "bar"))
True
>>> "foobar".find("ob")
2
>>> "ob".index("kek")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: substring not found
>>> "boo" in b"foobar"
# ???
>>> b"foobar".replace("o", "")
# ???

---

7️⃣ Replacing things

>>> "clubclubhouse".replace("club", "**", 1)
'**clubhouse'
>>> translation_map = {
  ord("C"): "B",
  ord("L"): "E",
  ord("U"): "E",
  ord("B"): "R"
}
>>> "CLUBhouse".translate(translation_map)
'BEERhouse'

---

8️⃣ Useful predicates

>>> "100500".isdigit()
True
>>> "foo100500".isalnum()
True
>>> "foobar".isalpha()
True
>>> "foobar".islower()
True
>>> "FOOBAR".isupper()
True
>>> "Foo Bar".istitle()
True
>>> "\r \n\t \r\n".isspace()
True

---

9️⃣ Formatting

>>> cost = 300
>>> f"You owe me {cost} bucks"
'You owe me 300 bucks'
>>> f"{3000000:,}"
'3,000,000'
>>> f"{3000000:~^11}"
'~~3000000~~'
>>> "{0}, {1}, {0}".format("hello", "kitty")
'hello, kitty, hello'
>>> "{0}, {who}, {0}".format("hello", who="kitty")
'hello, kitty, hello'

FYI: you don’t need to remember formatting symbols (use docs!)

---

Formatting: please, don’t

>>> "%s, %s, how are you?" % ("Hello", "Sally")
'Hello, Sally, how are you?'

• % is binary, requires list/tuple
• every element is used only once
• problems with long numbers

---

import string

>>> string.ascii_letters
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
>>> string.digits
'0123456789'
>>> string.punctuation
'!"#%&\'()*+,-./:;<=>?@[\\]^_`{|}~'
>>> string.whitespace
' \t\n\r\x0b\x0c'

---

Strings and bytes wrap-up

• strings are sequences of codepoints (Unicode sequences), symbol is a string too
• cool methods for bytes and strings
• Universal Character Set: N bytes for symbol (1, 2, 4)
• don’t use %, use f"" and"".forma
• do not confuse bytes and strings

---

📂 Files and IO

---

🚀 Files opening

>>> open("./memes.txt")  # text
<[...] name='./memes.txt' mode='r' encoding='UTF-8'>

modes: “r”, “w”, “x”, “a”, ”+”, “b”, “t”.

>>> open("./binary.db", "r+b")  # ???
<_io.BufferedRandom name='./binary.db'>

---

Reading and writing

>>> handle = open("./memes.txt")
>>> handle.read(13)
'HEREWEGOAGAIN'
>>> handle.readline(13)
'HEREWEGOAGAIN'
>>> handle.close()
>>> handle = open("./example.txt", "w")
>>> handle.write("abracadabra")
11 # written bytes

---

Other functions

>>> handle = open("./example.txt", "r+")
>>> handle.fileno()
3  # ???
>>> handle.tell()
0
>>> handle.seek(8)
>>> handle.tell()
8
>>> handle.write("something unimportant")
>>> handle.flush()
>>> handle.close()

---

🎶 Three default text files

>>> input("Name: ")  # read from sys.stdin
Name: Floppa
'Floppa'
>>> print("Hello, `sys.stdout`!", file=sys.stdout)
Hello, `sys.stdout`!
>>> print("Hello, `sys.stderr`!", file=sys.stderr)
Hello, `sys.stderr`!

---

🆕 from pathlib import Path

>>> Path(".") / "hello.txt"
PosixPath('hello.txt')
>>> p = Path("hello.txt")
>>> p.parent
PosixPath('.')
>>> p.resolve()
PosixPath('/Users/tv/hello.txt')
>>> p.write_text("hello")
5
>>> p.read_text()
'hello'
>>> p.unlink()
>>> p.read_text()
...
FileNotFoundError: [Errno 2] No such file or directory: 'hello.txt'

---

🔥 Files and IO wrap-up

• files can be texts or bytes
• methods of reading writing are typical
• Python has stdin, stdout and stderr – text files
• methods are typical, pathlib is useful

---

Collections

---

set

>>> xs, ys, zs = {1, 2}, {2, 3}, {3, 4}
>>> set.union(xs, ys, zs) # xs | ys | zs
{1, 2, 3, 4}
>>> set.intersection(xs, ys, zs) # xs & ys & zs
set()
>>> set.difference(xs, ys, zs) # xs - ys - zs
{1}
>>> xs <= ys # xs ⊆ ys
False
>>> xs < xs # xs ⊂ xs
False
>>> xs | ys >= xs # xs ∪ ys ⊇ xs
True

---

>>> seen = set()
>>> seen.add(42)
>>> seen
{42}
>>> seen.update([], [1], [2], [3])
>>> seen
{1, 2, 42, 3}
>>> seen.remove(3)
>>> seen
{1, 2, 42}
>>> seen.remove(100500) # ???
>>> {set(), set()} # ???

---

frozenset

Supports all set operations except adding/removing elements.

>>> {frozenset(), frozenset()}
{frozenset()}

---

tuple

>>> p = (5,) # ???
>>> date = "October", 5
>>> person = ("George", "Carlin", "May", 12, 1937)
>>> name, birthday = person[:2], person[2:]
>>> name
('George', 'Carlin')
>>> NAME, BIRTHDAY = slice(2), slice(2, None)
(None, 2, None)
>>> person[BIRTHDAY]
('May', 12, 1937)

---

namedtuple

>>> from collections import namedtuple
>>> Person = namedtuple("Person", ["name", "age"])
>>> p = Person("George", age=77)
>>> p._fields
('name', 'age')
>>> p.name, p.age
('George', 77)
>>> p._asdict()
OrderedDict([('name', 'George'), ('age', 77)])
>>> p._replace(name="Bill")
Person(name='Bill', age=77)

---

list

>>> [0] * 2
[0, 0]
>>> chunks = [[0]] * 2
>>> chunks
[[0], [0]]
>>> chunks[0][0] = 42
>>> chunks
[[42], [42]] # ???
>>> xs = [3, 2, 1]
>>> xs.sort(key=lambda x: x % 2, reverse=True)  # timsort
>>> xs
[3, 1, 2]

---

>>> xs = [1, 2, 3]
>>> xs.append(42) # ==> [1, 2, 3, 42]
>>> xs.extend({-1, -2}) # ==> [1, 2, 3, 42, -2, -1]
>>> xs = [1, 2, 3]
>>> xs.insert(0, 4) # ==> [4, 1, 2, 3]
>>> xs.insert(-1, 42) # ==> [4, 1, 2, 42, 3]
>>> xs = [1, 2, 3]
>>> xs[:2] = [0] * 2 # ==> [0, 0, 3]
>>> xs = [1, 2, 3]
>>> del xs[:2]
>>> xs.pop(1)
2  # sometimes it's useful
>>> xs = [1, 1, 0]
>>> xs.remove(1)
# ???

---

tuple and list

• reversed()
• concatenation: x + y (always a new object)

>>> xs, ys = [1, 2], [3]
>>> id(xs), id(ys)
(4416336136, 4416336008)
>>> id(xs + ys)
4415376136

---

but inplace concatenation in list!

>>> xs += ys # ≈ xs = xs.extend(ys)
>>> id(xs)
4416336136

by the way…

>>> xs = []
>>> def f():
        xs += [42]
...
>>> f()  # ???

---

dict

>>> d = dict(foo="bar")
>>> dict(d) # copy
{'foo': 'bar'}
>>> dict(d, boo="baz") # copy with new keys
{'boo': 'baz', 'foo': 'bar'}
>>> dict.fromkeys("abcd", 0)
{'d': 0, 'a': 0, 'b': 0, 'c': 0}
>>> d = dict.fromkeys(["foo", "bar"], 42)
>>> d.keys()
dict_keys(['foo', 'bar'])
>>> d.values()
dict_values([42, 42])
>>> d.items()
dict_items([('foo', 42), ('bar', 42)])

---

>>> for k in d:
      del d[k]
...
# ???
>>> len(d.items())
2
>>> 42 in d.values()
True
>>> d.keys() & {"foo"}
{'foo'}
>>> {v for v in d.values()}
{42}

---

>>> d = {"foo": "bar"}
>>> d["foo"]
'bar'
>>> d["boo"]
# ???

---

>>> d = {"foo": "bar"}
>>> d["foo"]
'bar'
>>> d["boo"]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'boo'
>>> d.get("boo", 42)
42

---

>>> d = {}
>>> d.update([("foo", "bar")], boo=42)
>>> d
{'boo': 42, 'foo': 'bar'}
>>> del d["boo"]
>>> d.pop("foo")
'bar'
>>> d.clear()
>>> d
{}

---

🏁 Any questions?