168 lines
3.9 KiB
Markdown
168 lines
3.9 KiB
Markdown
---
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title: Python
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category: Python
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---
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### Tuples (immutable)
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tuple = ()
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### Lists (mutable)
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list = []
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list[i:j] # returns list subset
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list[-1] # returns last element
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list[:-1] # returns all but the last element
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*list # expands all elements in place
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list[i] = val
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list[i:j] = otherlist # replace ith to jth-1 elements with otherlist
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del list[i:j]
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list.append(item)
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list.extend(another_list)
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list.insert(index, item)
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list.pop() # returns and removes last element from the list
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list.pop(i) # returns and removes i-th element from the list
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list.remove(i) # removes the first item from the list whose value is i
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list1 + list2 # combine two list
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set(list) # remove duplicate elements from a list
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list.reverse() # reverses the elements of the list in-place
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list.count(item)
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sum(list)
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zip(list1, list2) # returns list of tuples with n-th element of both list1 and list2
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list.sort() # sorts in-place, returns None
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sorted(list) # returns sorted copy of list
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",".join(list) # returns a string with list elements seperated by comma
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### Dict
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dict = {}
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dict.keys()
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dict.values()
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"key" in dict # let's say this returns False, then...
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dict["key"] # ...this raises KeyError
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dict.get("key") # ...this returns None
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dict.setdefault("key", 1)
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**dict # expands all k/v pairs in place
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### Iteration
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for item in ["a", "b", "c"]:
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for i in range(4): # 0 to 3
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for i in range(4, 8): # 4 to 7
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for i in range(1, 9, 2): # 1, 3, 5, 7
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for key, val in dict.items():
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for index, item in enumerate(list):
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### [String](https://docs.python.org/2/library/stdtypes.html#string-methods)
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str[0:4]
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len(str)
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string.replace("-", " ")
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",".join(list)
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"hi {0}".format('j')
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f"hi {name}" # same as "hi {}".format('name')
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str.find(",")
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str.index(",") # same, but raises IndexError
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str.count(",")
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str.split(",")
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str.lower()
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str.upper()
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str.title()
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str.lstrip()
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str.rstrip()
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str.strip()
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str.islower()
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/* escape characters */
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>>> 'doesn\'t' # use \' to escape the single quote...
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"doesn't"
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>>> "doesn't" # ...or use double quotes instead
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"doesn't"
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>>> '"Yes," they said.'
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'"Yes," they said.'
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>>> "\"Yes,\" they said."
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'"Yes," they said.'
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>>> '"Isn\'t," they said.'
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'"Isn\'t," they said.'
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### Casting
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int(str)
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float(str)
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str(int)
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str(float)
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'string'.encode()
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### Comprehensions
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[fn(i) for i in list] # .map
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map(fn, list) # .map, returns iterator
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filter(fn, list) # .filter, returns iterator
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[fn(i) for i in list if i > 0] # .filter.map
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### Regex
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import re
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re.match(r'^[aeiou]', str)
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re.sub(r'^[aeiou]', '?', str)
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re.sub(r'(xyz)', r'\1', str)
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expr = re.compile(r'^...$')
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expr.match(...)
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expr.sub(...)
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## File manipulation
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### Reading
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```py
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file = open("hello.txt", "r") # open in read mode 'r'
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file.close()
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```
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```py
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print(file.read()) # read the entire file and set the cursor at the end of file
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print file.readline() # Reading one line
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file.seek(0, 0) # place the cursor at the beginning of the file
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```
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### Writing (overwrite)
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```py
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file = open("hello.txt", "w") # open in write mode 'w'
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file.write("Hello World")
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text_lines = ["First line", "Second line", "Last line"]
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file.writelines(text_lines)
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file.close()
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```
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### Writing (append)
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```py
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file = open("Hello.txt", "a") # open in append mode
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file.write("Hello World again")
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file.close()
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```
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### Context manager
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```py
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with open("welcome.txt", "r") as file:
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# 'file' refers directly to "welcome.txt"
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data = file.read()
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# It closes the file automatically at the end of scope, no need for `file.close()`.
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```
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