---
# System prepended metadata

title: Midterm 1 Key Concepts - Python Cheat Sheet
tags: [CSE6040]

---

# Python Cheat Sheet for Midterm 1
---
## General Syntax
| Operations | Result |
|----------|----------|
| x in s  | True if an item of s is equal to x, else False   |
| x not in s   | False if an item of s is equal to x, else True   |
| s + t   | Concatenation of s and t   |
| $s\times n$ or $n \times s$   |  Equivalent to adding s to itself n times  |
|  s[i]  |  ith item of s, where the origin is 0  |
|  s[i:j]  |  slice of s from i to j  |
|  s[i:j:k]  |  slice of s from i to j with step k  |
|  len(s)  |  Length of s, where the origin is 1 (i+1)  |
|  min(s)  |  smallest item of s  |
|  max(s)  |  largest item of s  |
|  s.index(x[,i[,j]])  |  Index of the first occurrence of x in s at or after i but before j  |
|  s.count(x)  |  total # of occurrences of x in s  |

## Tuples
* Ordered - Elements are sequentially arranged according to insertion order
* Indexable - Access elements using an index
* Immutable - Cannot be changed
* Heterogeneous - Can store objects of different data types
* Nestable - Can contain other tuples
* Iterable - Traverse using a loop or comprehensions while you perform operations
* Sliceable - Extract some sequential elements
* Combinable - Support concatenations
* Hashable - Can work as keys in dictionaries
* 
### Useful Python Code for Tuples

## Lists
* Ordered – Elements are arranged in insertion order
* Indexable – Access elements with an index (L[0])
* Mutable – Can be changed (add, remove, modify elements)
* Heterogeneous – Can store different data types
* Nestable – Can contain other lists (or mixed structures)
* Iterable – Traverse using loops or comprehensions
* Sliceable – Extract subsequences (L[1:4])
* Combinable – Support concatenation (L1 + L2) and repetition (L * 3)
* Duplicate-friendly – Allows repeated elements

### Useful Python Code for Lists
* Append vs Extend - use append to add a single item to the end of a list, use extend to add an iterable (list, string, set, etc) to the end of a list:
** Refer to PMT1-F22Q2
```Python
L = [1, 2]
L.append(3)
# [1, 2, 3]
L.extend([3, 4])
# [1, 2, 3, 4]
```

## Dictionaries (and Defaults)
* Unordered (3.6+ ordered by insertion) – Keys maintain insertion order (Python ≥3.7 officially guarantees this)
* Key–Value pairs – Store data as mappings {key: value}
* Mutable – Values can be added, updated, or deleted
* Key restrictions – Keys must be immutable (e.g., str, int, tuple), values can be any type
* Unique keys – Duplicate keys not allowed (last assignment wins)
* Iterable – Iterate over keys, values, or items (d.keys(), d.values(), d.items())
* Efficient lookup – Fast access by key (d["name"])
* Nestable – Can contain lists, dicts, or sets as values
* Hashable keys – Only hashable objects can be keys
* To access the keys, values, and items within a dictionary:
```Python
keys = dictionary_name.keys() #returns a list of keys
values = dictionary_name.values() #returns a list of values
items = dictionary_name.items() #returns a list of tuples (key, value)
```
* To default search for a value using a key:
```Python
value = dictionary_name["key"]
```
#### Default Dictionary Syntax
```from collections import defaultdict

# Create with a "default factory" function
d = defaultdict(int)         # default 0 for missing keys
d = defaultdict(list)        # default [] for missing keys
d = defaultdict(set)         # default set() for missing keys```

### Useful Python Code for Dictionaries
* To add an item to a dictionary: 
```Python
dictionary_name['key'] = 'value'
```


## Sets
* Unordered – No defined element order
* Unindexed – Cannot access by index (s[0] invalid)
* Mutable – Can add or remove elements (add, discard, remove)
* Unique elements – Automatically removes duplicates
* Heterogeneous – Can store different data types (as long as they’re hashable)
* Nestable restriction – Cannot contain mutable objects like lists or dicts, but can contain tuples
* Iterable – Traverse elements using loops
* Set operations – Union (|), Intersection (&), Difference (-), Symmetric difference (^)
* Efficient membership test – Fast in checks

### Useful Python Code for Sets
* & between two sets = intersection (the elements they share).
* | = union (all elements from both).
* \- = difference (things in the first but not the second).
* ^ = symmetric difference (things in one or the other, but not both).
## JSON Files
## Nested Data Types
* PMT1-Fall22Q1 - Contains a Reddit question that displays a JSON file that is a large amount of nested list and dictionaries. 
## Regex
#### Character Classes
* \d → digit (0–9)
* \D → non-digit (anything not 0–9)
* \w → “word” character = [A-Za-z0-9_]
* \W → non-word character
* \s → whitespace (space, tab, newline, etc.)
* \S → non-whitespace
#### Quantifiers
* * → 0 or more
* + → 1 or more
* ? → 0 or 1 (optional)
* {n} → exactly n
* {n,} → at least n
* {n,m} → between n and m
#### Anchors / position
* ^ → start of string (or line, with re.M)
* $ → end of string (or line, with re.M)
* \b → word boundary
* \B → not a word boundary
#### Wildcards & groups
* . → any single character except newline (unless re.S)
* ( … ) → capturing group
* (?: … ) → non-capturing group
* (?P<name> … ) → named group
* | → OR
#### Functions
| Function | What it does | Example
|----------|----------|----------
| re.search(pattern, text) | Finds the first match anywhere in the string. Returns a Match object or None. | ```re.search(r"\d+", "abc123xyz").group() → "123" ``` |
| re.match(pattern, text) | Like search, but only checks at the start of the string. | ```re.match(r"\w+", "hello world").group() → "hello"``` |
| re.findall(pattern, text) | Returns a list of all matches. | ```re.findall(r"\d+", "a1 b22 c333") → ['1', '22', '333']``` |
| re.finditer(pattern, text) | Returns an iterator of Match objects (useful for groups/named groups). | ```[m.group() for m in re.finditer(r"\d+", "a1 b22")] → ['1','22']``` |
| re.sub(pattern, repl, text) | Replace all matches with repl. | ```re.sub(r"\d+", "#", "a1 b22") → "a# b#"``` |
| re.split(pattern, text) | Split string by the pattern (like str.split but regex-aware). | ```re.split(r"\s+", "a b c") → ['a','b','c']``` |
| re.compile(pattern, flags=0) | Precompile regex for reuse; attach methods like .search(), .findall(). | ```pat = re.compile(r"\d+"); pat.findall("a1 b22") → ['1','22']``` |
#### Useful Match Object Methods
* .group() → whole match or group by index/name.
* .groups() → tuple of all capture groups.
* .groupdict() → dict of named groups.
* .start(), .end() → positions of match in string.
#### Flags

| Flag | Name | Modification |
|------|------|------|
| ```re.I``` | ignore casing | Makes the expression search case-insensitive |
| ```re.G``` | global | Makes the expression search for all occurrences. |
| ```re.S``` | dot all | Makes the wild character ```.``` match newlines as well. |
| ```re.M``` | multiline | Makes the bounday characters ```^``` and ```$``` match the beginning and ending of every single line instead of the beginning and ending of the whole string. |
| ```re.Y``` | sticky | Makes the expression start its searching from the index indicated in its ```lastIndex``` property. |
| ```re.U``` | unicode | Makes the expression assume individual characters as code points, not code units, and thus match 32-bit characters as well. |


## Strings
* ```text = text.lower()``` for lower case
* ```text = text.upper()``` for upper case
* ```text = " ".join()``` to concatenate items in an iterable into a single string
* ```text.split()``` splits a string on whitespace (" ", \n)
* ```text = " ".join(text.split())``` to collapse any run of whitespace down to a single space
* ```text.strip()``` removes white space by default, but can specify a string/character to remove
* ```text.replace(old, new)``` replaces occurrences of the old with the new
* ```f"{var}"``` f-string syntax

| Function         | Returns True if…                                         | Example                         |
|------------------|----------------------------------------------------------|---------------------------------|
| `str.isalpha()`  | All characters are alphabetic (A–Z, a–z)                 | `"abc".isalpha()` → True        |
| `str.isdigit()`  | All characters are digits (0–9)                          | `"123".isdigit()" → True        |
| `str.isnumeric()`| All characters are numeric (digits + fractions, etc.)    | `"Ⅻ".isnumeric()` → True        |
| `str.isdecimal()`| All characters are decimal digits only                   | `"123".isdecimal()" → True      |
| `str.isalnum()`  | All characters are alphanumeric (letters + digits)       | `"abc123".isalnum()" → True     |
| `str.isspace()`  | All characters are whitespace (spaces, tabs, newlines)   | `"   \n".isspace()" → True      |
| `str.islower()`  | All cased characters are lowercase (and at least one)    | `"hello".islower()" → True      |
| `str.isupper()`  | All cased characters are uppercase (and at least one)    | `"HELLO".isupper()" → True      |
| `str.istitle()`  | String is titlecased (e.g., “Hello World”)               | `"Hello World".istitle()" → True|
| `str.isidentifier()` | String is a valid Python identifier                  | `"variable1".isidentifier()" → True |
| `str.isprintable()`  | All characters are printable or spaces                | `"abc!".isprintable()" → True   |

