Python Quick Reference

1name = "Sara"            # str  (text)
2age = 29                 # int  (whole number)
3height = 1.68            # float (decimal)
4member = True            # bool (True or False)
5nothing = None           # no value at all
6
7type(age)                # <class 'int'>
8int("42"), float("3.5"), str(42)    # converting

1f"{name} is {age}"       # f-string
2f"{price:.2f}"           # two decimal places
3len(name)                # 4
4name.upper(), name.lower(), name.title()
5name.strip()             # remove spaces at both ends
6name.replace("a", "o")
7name.split(",")          # text -> list
8", ".join(names)         # list -> text
9name.startswith("S"), name.endswith(".py")
10"ar" in name             # True
11name[0], name[-1], name[1:3]

17 + 2, 7 - 2, 7 * 2, 7 / 2     # 9 5 14 3.5
27 // 2, 7 % 2, 7 ** 2          # 3 1 49
3round(3.14159, 2)              # 3.14
4==  !=  <  <=  >  >=           # comparisons
5and  or  not                   # combining them

1if score >= 80:
2    print("great")
3elif score >= 50:
4    print("pass")
5else:
6    print("try again")
7
8for item in items:
9    print(item)
10
11for i in range(5):             # 0, 1, 2, 3, 4
12    print(i)
13
14for i, item in enumerate(items, start=1):
15    print(i, item)
16
17while count > 0:
18    count -= 1
19
20break                          # leave the loop now
21continue                       # skip to the next round

1items = ["a", "b", "c"]
2items.append("d"); items.remove("a"); items.sort()
3items[0], items[-1], items[1:3], len(items)
4point = (3, 4)                       # tuple: can't change
5phones = {"Ana": "0412", "Raj": "0499"}
6phones["Zoe"] = "0455"               # add or change
7phones.get("Tom", "unknown")         # safe lookup
8for name, number in phones.items():
9    print(name, number)
10seen = {"red", "blue"}               # set: no duplicates
11seen.add("red")                      # still two items
12squares = [n * n for n in range(5)]  # list comprehension

1def area(width, height=1):
2    """Return the area of a rectangle."""
3    return width * height
4
5area(3, 4)
6area(width=3)                  # height uses its default

1try:
2    age = int(text)
3except ValueError:
4    print("Please type a number.")
5else:
6    print("Thanks")
7finally:
8    print("This always runs")
9
10raise ValueError("age must be positive")

1from pathlib import Path
2
3HERE = Path(__file__).parent
4path = HERE / "data" / "notes.txt"
5text = path.read_text(encoding="utf-8")
6path.write_text("hello\n", encoding="utf-8")
7
8with open(path, "a", encoding="utf-8") as f:
9    f.write("one more line\n")
10
11path.exists(), path.name, path.stem, path.suffix
12for p in HERE.glob("*.csv"):
13    print(p.name)

1class Remote:
2    MAX_VOLUME = 100                 # class variable
3
4    def __init__(self, name):        # factory settings
5        self.name = name             # instance variable
6        self.volume = 10
7
8    def volume_up(self):             # a method
9        self.volume += 1
10
11    def __str__(self):               # used by print()
12        return f"{self.name}: {self.volume}"
13
14bedroom = Remote("Bedroom")          # an object
15bedroom.volume_up()

1import math                    # use as math.sqrt(16)
2from random import choice      # use as choice(items)
3import pandas as pd            # a short nickname
4
5if __name__ == "__main__":     # only when run directly
6    main()