What values are valid in Pandas “Freq” tags?

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I am new to Pandas, and am trying to use date_range. I came across all kinds of good things for freq, like BME and BMS and I would like to be able to quickly look up the proper strings to get what I want. Yesterday I found a nicely formatted table somewhere in the documentation, but the title of the table was so obtuse that I can not use search to find it again today.

What values are valid in Pandas "Freq" tags?

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What values are valid in Pandas "Freq" tags? date_range: Questions

How do I filter query objects by date range in Django?

2 answers

I"ve got a field in one model like:

class Sample(models.Model):
    date = fields.DateField(auto_now=False)

Now, I need to filter the objects by a date range.

How do I filter all the objects that have a date between 1-Jan-2011 and 31-Jan-2011?

299

Answer #1

Use

Sample.objects.filter(date__range=["2011-01-01", "2011-01-31"])

Or if you are just trying to filter month wise:

Sample.objects.filter(date__year="2011", 
                      date__month="01")

Edit

As Bernhard Vallant said, if you want a queryset which excludes the specified range ends you should consider his solution, which utilizes gt/lt (greater-than/less-than).

299

Answer #2

You can use django"s filter with datetime.date objects:

import datetime
samples = Sample.objects.filter(sampledate__gte=datetime.date(2011, 1, 1),
                                sampledate__lte=datetime.date(2011, 1, 31))

Finding the index of an item in a list

5 answers

Given a list ["foo", "bar", "baz"] and an item in the list "bar", how do I get its index (1) in Python?

3740

Answer #1

>>> ["foo", "bar", "baz"].index("bar")
1

Reference: Data Structures > More on Lists

Caveats follow

Note that while this is perhaps the cleanest way to answer the question as asked, index is a rather weak component of the list API, and I can"t remember the last time I used it in anger. It"s been pointed out to me in the comments that because this answer is heavily referenced, it should be made more complete. Some caveats about list.index follow. It is probably worth initially taking a look at the documentation for it:

list.index(x[, start[, end]])

Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError if there is no such item.

The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.

Linear time-complexity in list length

An index call checks every element of the list in order, until it finds a match. If your list is long, and you don"t know roughly where in the list it occurs, this search could become a bottleneck. In that case, you should consider a different data structure. Note that if you know roughly where to find the match, you can give index a hint. For instance, in this snippet, l.index(999_999, 999_990, 1_000_000) is roughly five orders of magnitude faster than straight l.index(999_999), because the former only has to search 10 entries, while the latter searches a million:

>>> import timeit
>>> timeit.timeit("l.index(999_999)", setup="l = list(range(0, 1_000_000))", number=1000)
9.356267921015387
>>> timeit.timeit("l.index(999_999, 999_990, 1_000_000)", setup="l = list(range(0, 1_000_000))", number=1000)
0.0004404920036904514
 

Only returns the index of the first match to its argument

A call to index searches through the list in order until it finds a match, and stops there. If you expect to need indices of more matches, you should use a list comprehension, or generator expression.

>>> [1, 1].index(1)
0
>>> [i for i, e in enumerate([1, 2, 1]) if e == 1]
[0, 2]
>>> g = (i for i, e in enumerate([1, 2, 1]) if e == 1)
>>> next(g)
0
>>> next(g)
2

Most places where I once would have used index, I now use a list comprehension or generator expression because they"re more generalizable. So if you"re considering reaching for index, take a look at these excellent Python features.

Throws if element not present in list

A call to index results in a ValueError if the item"s not present.

>>> [1, 1].index(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: 2 is not in list

If the item might not be present in the list, you should either

  1. Check for it first with item in my_list (clean, readable approach), or
  2. Wrap the index call in a try/except block which catches ValueError (probably faster, at least when the list to search is long, and the item is usually present.)

3740

Answer #2

One thing that is really helpful in learning Python is to use the interactive help function:

>>> help(["foo", "bar", "baz"])
Help on list object:

class list(object)
 ...

 |
 |  index(...)
 |      L.index(value, [start, [stop]]) -> integer -- return first index of value
 |

which will often lead you to the method you are looking for.

3740

Answer #3

The majority of answers explain how to find a single index, but their methods do not return multiple indexes if the item is in the list multiple times. Use enumerate():

for i, j in enumerate(["foo", "bar", "baz"]):
    if j == "bar":
        print(i)

The index() function only returns the first occurrence, while enumerate() returns all occurrences.

As a list comprehension:

[i for i, j in enumerate(["foo", "bar", "baz"]) if j == "bar"]

Here"s also another small solution with itertools.count() (which is pretty much the same approach as enumerate):

from itertools import izip as zip, count # izip for maximum efficiency
[i for i, j in zip(count(), ["foo", "bar", "baz"]) if j == "bar"]

This is more efficient for larger lists than using enumerate():

$ python -m timeit -s "from itertools import izip as zip, count" "[i for i, j in zip(count(), ["foo", "bar", "baz"]*500) if j == "bar"]"
10000 loops, best of 3: 174 usec per loop
$ python -m timeit "[i for i, j in enumerate(["foo", "bar", "baz"]*500) if j == "bar"]"
10000 loops, best of 3: 196 usec per loop

We hope this article has helped you to resolve the problem. Apart from What values are valid in Pandas “Freq” tags?, check other date_range-related topics.

Want to excel in Python? See our review of the best Python online courses 2022. If you are interested in Data Science, check also how to learn programming in R.

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Dmitry Chamberlet

San Francisco | 2022-12-05

StackOverflow is always a bit confusing 😭 What values are valid in Pandas “Freq” tags? is not the only problem I encountered. I am just not quite sure it is the best method

Walter Danburry

Milan | 2022-12-05

Maybe there are another answers? What What values are valid in Pandas “Freq” tags? exactly means?. I just hope that will not emerge anymore

Julia Sikorski

Paris | 2022-12-05

Simply put and clear. Thank you for sharing. What values are valid in Pandas “Freq” tags? and other issues with find was always my weak point 😁. I am just not quite sure it is the best method

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