One google behind this stackoverflow result, I found that there is a numpy.fromiter(data, dtype, count). The default count=-1 takes all elements from the iterable. It requires a dtype to be set explicitly. In my case, this worked: numpy.fromiter(something.generate(from_this_input), float)
Quick answer: Doing list() around a generator expression is (almost) exactly equivalent to having [] brackets around it. So yeah, you can do >>> list((x for x in string.letters if x in (y for y in “BigMan on campus”))) But you can just as well do >>> [x for x in string.letters if x in … Read more
>>> from itertools import groupby >>> L = [0, 0, 0, 3, 3, 2, 5, 2, 6, 6] >>> grouped_L = [(k, sum(1 for i in g)) for k,g in groupby(L)] >>> # Or (k, len(list(g))), but that creates an intermediate list >>> grouped_L [(0, 3), (3, 2), (2, 1), (5, 1), (2, 1), (6, … Read more
Hate to say this, but MySQL is the only RDBMS of the big four that doesn’t have this feature. In Oracle: SELECT * FROM dual CONNECT BY level < n In MS SQL (up to 100 rows): WITH hier(row) AS ( SELECT 1 UNION ALL SELECT row + 1 FROM hier WHERE row < n … Read more
Generators can’t be rewound. You have the following options: Run the generator function again, restarting the generation: y = FunctionWithYield() for x in y: print(x) y = FunctionWithYield() for x in y: print(x) Store the generator results in a data structure on memory or disk which you can iterate over again: y = list(FunctionWithYield()) for … Read more
I would recommend looking at David Beazley’s Generator Tricks for Python, especially Part 5: Processing Infinite Data. It will handle the Python equivalent of a tail -f logfile command in real-time. # follow.py # # Follow a file like tail -f. import time def follow(thefile): thefile.seek(0,2) while True: line = thefile.readline() if not line: time.sleep(0.1) … Read more
Suggestion: def peek(iterable): try: first = next(iterable) except StopIteration: return None return first, itertools.chain([first], iterable) Usage: res = peek(mysequence) if res is None: # sequence is empty. Do stuff. else: first, mysequence = res # Do something with first, maybe? # Then iterate over the sequence: for element in mysequence: # etc.
Slicing a list top5 = array[:5] To slice a list, there’s a simple syntax: array[start:stop:step] You can omit any parameter. These are all valid: array[start:], array[:stop], array[::step] Slicing a generator import itertools top5 = itertools.islice(my_list, 5) # grab the first five elements You can’t slice a generator directly in Python. itertools.islice() will wrap an object … Read more
Generators give you lazy evaluation. You use them by iterating over them, either explicitly with ‘for’ or implicitly by passing it to any function or construct that iterates. You can think of generators as returning multiple items, as if they return a list, but instead of returning them all at once they return them one-by-one, … Read more
It’s used to send values into a generator that just yielded. Here is an artificial (non-useful) explanatory example: >>> def double_inputs(): … while True: … x = yield … yield x * 2 … >>> gen = double_inputs() >>> next(gen) # run up to the first yield >>> gen.send(10) # goes into ‘x’ variable 20 … Read more