Dictionary
- A collection of data for Key-value key-value pairs
- Variable, unordered, key non-repeating
Initialization
d = dict()d = {}d = dict(**kwargs) 如：d = dict(a=1,b=2)dict(iterable, **kwarg)
Constructs a dictionary using an iterative object and a Name=value pair, but the elements of an iterative object must be a two-tuple structure
dict(mapping, **kwarg)dict.fromkeys(iterable, value)
Access to dictionary elements
- D[key]
- Returns the value corresponding to key
- Key does not exist throwing Keyerror exception
- Get (key[, default])
- Returns the value corresponding to key
- Key does not exist return default value, return None if default value is not set
- SetDefault (key[, default])
- Returns the value corresponding to key
- Key does not exist, adds a kv pair, value is default, and returns default if default is not set, default is None
Dictionary Additions and modifications
- D[key] = value
- Change the value corresponding to key to value
- Key does not exist adding a new kv pair
- Update ([other]), None
- Update this dictionary with the kv pair of another dictionary
- Key does not exist, just add
- Key exists, overwriting the value corresponding to the existing key
- In-place modification
d.update(red=1)d.update(((‘red‘,2),))d.update({‘red‘:3})
Dictionary Delete
- Pop (key[, default])
- Key exists, removes it, and returns its value
- Key does not exist, returns the given default
- Default is not set, key does not exist throw Keyerror exception
Popitem ()
- Removes and returns an arbitrary key-value pair
- The dictionary is empty and throws a Keyerror exception
- Clear ()
- Empty dictionary
- Del statement
- Essentially reduces the reference to an object, Del actually removes the name, not the object
Dictionary traversal:
for k in d:print(k)for k in d.keys():print(k)
for k in d:print(d[k])for k in d:print(d.get(k))for v in d.values():print(v)
* traverse Item, that is, KV pair:
for item in d.items():print(item)for item in d.items():print(item[0], item[1])for k,v in d.items():print(k, v)for k, _ in d.items():print(k)for _ ,v in d.items():print(v)
Summarize
- In Python3, the keys, values, items method returns an iterative object that resembles a generator, and does not copy the return result of the function into memory
- Dictionary View Object
- The entry dynamic view of the dictionary, the dictionary changes, the view will reflect these changes
Dictionary traversal and Removal
- Adding or deleting elements is not allowed when traversing a dictionary
- You can log the key value before traversing delete
d = dict(a=1, b=2, c=‘abc‘)keys = []for k,v in d.items():if isinstance(v, str):keys.append(k)for k in keys:d.pop(k)print(d)
Defaultdict
- Collections.defaultdict ([default_factory[, ...])
- The first parameter is Default_factory, the default is None, and it provides an initialization function. When the key does not exist, the factory function is called to generate the value corresponding to the key.
Ordereddict
- Collections. Ordereddict ([items])
- Keys are not listed in the order in which they are added, you can use the Ordereddict record order
Standard library DateTime DateTime module
- DateTime class
- Today ()
- Returns a DateTime object for the current time of the local time zone
- Now (Tz=none)
- Returns the DateTime object of the current time, in microseconds, if TZ is none, and returns to today ()
- Fromtimestamp (timestamp, tz=none) returns a DateTime object from a timestamp
- Date formatting
- Class method Strptime (date_string, format), returning a DateTime object
- Object method strftime (format), return string
- Formatting the string Format function
import datetimedt = datetime.datetime.strptime("21/11/06 16:30", "%d/%m/%y %H:%M")print(dt.strftime("%Y-%m-%d %H:%M:%S"))print("{0:%Y}/{0:%m}/{0:%d} {0:%H}::{0:%M}::{0:%S}".format(dt))
Timedelta Object
- Add and subtract from a DateTime object
- Datetime.timedelta (days=0, seconds=0, Microseconds=0, Milliseconds=0,minutes=0, hours=0, weeks=0)
- Total_seconds () returns the total number of seconds of the time difference
- Standard library Time
- Time.sleep (secs) suspends the calling thread for the specified number of seconds
List analytic type
Grammar
- [return value for element in iteration object if condition]
- Use brackets [], Inside is for loop, if conditional statement optional
- Returns a new list
- List parsing is a syntactic sugar
- The compiler optimizes, does not affect efficiency because of shorthand, but improves efficiency because of optimization
- Reduce programmer workload and reduce errors
- Simplified code, but improved readability
List Resolution Advanced
Builder expression
(return value for element in can iterate object if condition)
- The generator expression is calculated on demand (or lazy evaluation, lazy calculation), and the value is calculated when needed
- List parsing is an immediate return value
The generator is an iterative object, an iterator
- Using Next (itetator) iteration
Generator iterations vs. list iterations
Generator
- Deferred calculation
- Returns an iterator that can iterate
- You can't look back after you've walked through the past.
List
- Calculate Now
- Returns an iterator that returns a list of objects that can be iterated
- You can go back to the iteration after you've walked through the front and back again.
- Calculation method
- Generator expression Deferred calculation, list parsing immediate calculation
- Memory consumption
- Single from the return value itself, the generator expression saves memory, and the list resolves to return the new list
- The generator does not have data, memory consumption is very small, but when used, although the return of data, but combined to occupy the same amount of memory
- List parsing constructs a new list needs to occupy memory
- Calculation speed
- The generator expression takes a very short time to look at the calculation, and the list parsing is time-consuming
- But the generator itself does not return any values, only one generator object is returned
- List parsing constructs and returns a new list
Set Analytic type
- {The return value for element in can iterate object if condition}
- Returns a collection immediately
Dictionary analytic type
- {The return value for element in can iterate object if condition}
- Use Key:value form
- Returns a dictionary immediately
Built-in functions
- Identity ID
- Returns the unique identity of the object, CPython returns the memory address
- Hashed Hash ()
- Returns the hash value of an object
- Types Type ()
- Returns the type of the object
- Type conversions
float () int () bin () Hex () Oct () bool () list () tuple () Dict () set () complex () bytes () ByteArray ()
Enter input ([prompt])
- Receives user input, returns a string
- Printing print (*objects, sep= ", end= ' \ n ', File=sys.stdout, Flush=false)
- printout, default with space split, wrap end, output to console
- Object length Len (s)
- Returns the number of elements of a collection type
Isinstance (obj, class_or_tuple)
- Determines whether the object obj belongs to a type or a type listed in a tuple
- Isinstance (True, int)
Issubclass (CLS, class_or_tuple)
- Determines whether the type CLS is a subclass of a type or a subcategory of a type listed in a tuple
- Issubclass (bool, int)
- Absolute ABS (x)
- Max Max () min min ()
- Returns the maximum or minimum value in an object that can be iterated
- Returns the maximum or minimum value in multiple parameters
- Round (x)
- Four homes six into the five take, round (-0.5)
- Pow (x, y)
- Range (stop)
- An iterative object from 0 to Stop-1, range (start, stop[, step]) from start to stop-1 to end step as step
- Divmod (x, y)
- Equivalent to tuple (x//y, x%y)
Sum (iterable[, start])
- Sums all numeric elements of an object that can be iterated
- SUM (range (1,100,2))
- Chr (i)
- Returns the corresponding character to a range of integers
Ord (c)
- Returns the integer corresponding to the character
- Sorted (iterable[, key][, reverse]) sort
- Returns a new list, ascending by default
Reversed ()
Enumeration enumerate (SEQ, start=0)
- Iterates over a sequence that returns a two-tuple of indexed numbers and elements
*start A number that represents the start of the index, which is 0 by default
Iterators and fetching elements ITER (iterable), Next (iterator[, default])
- Iter encapsulates an iterative object into an iterator
- Next removes an element from an iterator. If all the elements have been taken, then next will throw a Stopiteration exception
- Zip function zip (*iterables)
- Like a zipper, a number of iterative objects are merged together to return an iterator
- Merges each element from a different object into a single tuple
- Bucket principle, take the minimum number of iterations iteration
Can iterate over objects
- The ability to iterate over and over again to return objects of different elements.
- The so-called same, does not mean whether the value is the same, but whether the element is the same in the container, such as the list of values can be repeated, [' A ', ' a '], although the list has 2 elements, the same value, but two ' a ' is a different element
- Can iterate, but not necessarily orderly, may not be indexed
- The object can be iterated: list, tuple, string, Bytes, ByteArray, range, set, dict, generator, etc.
- You can use the member operator in, not In,in is essentially traversing the object
Iterators
- A special object, which must be an iterative object with the characteristics of an iterative object
- To encapsulate an iterative object into an iterator using the ITER method
- Iterate through the iterator object with the next method
- The Builder object, which is the iterator object
Third week of Python study notes (1)
