1 # CONTRIBUTING TO YT-DLP
3 - [OPENING AN ISSUE](#opening-an-issue)
4 - [Is the description of the issue itself sufficient?](#is-the-description-of-the-issue-itself-sufficient)
5 - [Are you using the latest version?](#are-you-using-the-latest-version)
6 - [Is the issue already documented?](#is-the-issue-already-documented)
7 - [Why are existing options not enough?](#why-are-existing-options-not-enough)
8 - [Have you read and understood the changes, between youtube-dl and yt-dlp](#have-you-read-and-understood-the-changes-between-youtube-dl-and-yt-dlp)
9 - [Is there enough context in your bug report?](#is-there-enough-context-in-your-bug-report)
10 - [Does the issue involve one problem, and one problem only?](#does-the-issue-involve-one-problem-and-one-problem-only)
11 - [Is anyone going to need the feature?](#is-anyone-going-to-need-the-feature)
12 - [Is your question about yt-dlp?](#is-your-question-about-yt-dlp)
13 - [DEVELOPER INSTRUCTIONS](#developer-instructions)
14 - [Adding new feature or making overarching changes](#adding-new-feature-or-making-overarching-changes)
15 - [Adding support for a new site](#adding-support-for-a-new-site)
16 - [yt-dlp coding conventions](#yt-dlp-coding-conventions)
17 - [Mandatory and optional metafields](#mandatory-and-optional-metafields)
18 - [Provide fallbacks](#provide-fallbacks)
19 - [Regular expressions](#regular-expressions)
20 - [Long lines policy](#long-lines-policy)
21 - [Inline values](#inline-values)
22 - [Collapse fallbacks](#collapse-fallbacks)
23 - [Trailing parentheses](#trailing-parentheses)
24 - [Use convenience conversion and parsing functions](#use-convenience-conversion-and-parsing-functions)
25 - [EMBEDDING YT-DLP](README.md#embedding-yt-dlp)
31 Bugs and suggestions should be reported at:
[yt-dlp/yt-dlp/issues](https://github.com/yt-dlp/yt-dlp/issues). Unless you were prompted to or there is another pertinent reason (e.g. GitHub fails to accept the bug report), please do not send bug reports via personal email. For discussions, join us in our
[discord server](https://discord.gg/H5MNcFW63r).
33 **Please include the full output of yt-dlp when run with
`-Uv`**, i.e. **add**
`-Uv` flag to **your command line**, copy the **whole** output and post it in the issue body wrapped in \
`\`\
` for better formatting. It should look similar to this:
35 $ yt-dlp -Uv <your command line>
36 [debug] Command-line config: ['-v', 'demo.com']
37 [debug] Encodings: locale UTF-8, fs utf-8, out utf-8, pref UTF-8
38 [debug] yt-dlp version 2021.09.25 (zip)
39 [debug] Python version 3.8.10 (CPython 64bit) - Linux-5.4.0-74-generic-x86_64-with-glibc2.29
40 [debug] exe versions: ffmpeg 4.2.4, ffprobe 4.2.4
42 Current Build Hash 25cc412d1d3c0725a1f2f5b7e4682f6fb40e6d15f7024e96f7afd572e9919535
43 yt-dlp is up to date (2021.09.25)
46 **Do not post screenshots of verbose logs; only plain text is acceptable.**
48 The output (including the first lines) contains important debugging information. Issues without the full output are often not reproducible and therefore will be closed as `incomplete
`.
50 The templates provided for the Issues, should be completed and **not removed**, this helps aide the resolution of the issue.
52 Please re-read your issue once again to avoid a couple of common mistakes (you can and should use this as a checklist):
54 ### Is the description of the issue itself sufficient?
56 We often get issue reports that we cannot really decipher. While in most cases we eventually get the required information after asking back multiple times, this poses an unnecessary drain on our resources.
58 So please elaborate on what feature you are requesting, or what bug you want to be fixed. Make sure that it's obvious
61 - How it could be fixed
62 - How your proposed solution would look like
64 If your report is shorter than two lines, it is almost certainly missing some of these, which makes it hard for us to respond to it. We're often too polite to close the issue outright, but the missing info makes misinterpretation likely. We often get frustrated by these issues, since the only possible way for us to move forward on them is to ask for clarification over and over.
66 For bug reports, this means that your report should contain the **complete** output of yt-dlp when called with the `-Uv
` flag. The error message you get for (most) bugs even says so, but you would not believe how many of our bug reports do not contain this information.
68 If the error is `ERROR: Unable to extract ...
` and you cannot reproduce it from multiple countries, add `--write-pages
` and upload the `.dump
` files you get [somewhere](https://gist.github.com).
70 **Site support requests must contain an example URL**. An example URL is a URL you might want to download, like `https://www.youtube.com/watch?v=BaW_jenozKc
`. There should be an obvious video present. Except under very special circumstances, the main page of a video service (e.g. `https://www.youtube.com/
`) is *not* an example URL.
72 ### Are you using the latest version?
74 Before reporting any issue, type `yt-dlp -U
`. This should report that you're up-to-date. This goes for feature requests as well.
76 ### Is the issue already documented?
78 Make sure that someone has not already opened the issue you're trying to open. Search at the top of the window or browse the [GitHub Issues](https://github.com/yt-dlp/yt-dlp/search?type=Issues) of this repository. If there is an issue, feel free to write something along the lines of "This affects me as well, with version 2021.01.01. Here is some more information on the issue: ...". While some issues may be old, a new post into them often spurs rapid activity.
80 Additionally, it is also helpful to see if the issue has already been documented in the [youtube-dl issue tracker](https://github.com/ytdl-org/youtube-dl/issues). If similar issues have already been reported in youtube-dl (but not in our issue tracker), links to them can be included in your issue report here.
82 ### Why are existing options not enough?
84 Before requesting a new feature, please have a quick peek at [the list of supported options](README.md#usage-and-options). Many feature requests are for features that actually exist already! Please, absolutely do show off your work in the issue report and detail how the existing similar options do *not* solve your problem.
86 ### Have you read and understood the changes, between youtube-dl and yt-dlp
88 There are many changes between youtube-dl and yt-dlp [(changes to default behavior)](README.md#differences-in-default-behavior), and some of the options available have a different behaviour in yt-dlp, or have been removed all together [(list of changes to options)](README.md#deprecated-options). Make sure you have read and understand the differences in the options and how this may impact your downloads before opening an issue.
90 ### Is there enough context in your bug report?
92 People want to solve problems, and often think they do us a favor by breaking down their larger problems (e.g. wanting to skip already downloaded files) to a specific request (e.g. requesting us to look whether the file exists before downloading the info page). However, what often happens is that they break down the problem into two steps: One simple, and one impossible (or extremely complicated one).
94 We are then presented with a very complicated request when the original problem could be solved far easier, e.g. by recording the downloaded video IDs in a separate file. To avoid this, you must include the greater context where it is non-obvious. In particular, every feature request that does not consist of adding support for a new site should contain a use case scenario that explains in what situation the missing feature would be useful.
96 ### Does the issue involve one problem, and one problem only?
98 Some of our users seem to think there is a limit of issues they can or should open. There is no limit of issues they can or should open. While it may seem appealing to be able to dump all your issues into one ticket, that means that someone who solves one of your issues cannot mark the issue as closed. Typically, reporting a bunch of issues leads to the ticket lingering since nobody wants to attack that behemoth, until someone mercifully splits the issue into multiple ones.
100 In particular, every site support request issue should only pertain to services at one site (generally under a common domain, but always using the same backend technology). Do not request support for vimeo user videos, White house podcasts, and Google Plus pages in the same issue. Also, make sure that you don't post bug reports alongside feature requests. As a rule of thumb, a feature request does not include outputs of yt-dlp that are not immediately related to the feature at hand. Do not post reports of a network error alongside the request for a new video service.
102 ### Is anyone going to need the feature?
104 Only post features that you (or an incapacitated friend you can personally talk to) require. Do not post features because they seem like a good idea. If they are really useful, they will be requested by someone who requires them.
106 ### Is your question about yt-dlp?
108 Some bug reports are completely unrelated to yt-dlp and relate to a different, or even the reporter's own, application. Please make sure that you are actually using yt-dlp. If you are using a UI for yt-dlp, report the bug to the maintainer of the actual application providing the UI. In general, if you are unable to provide the verbose log, you should not be opening the issue here.
110 If the issue is with `youtube-dl
` (the upstream fork of yt-dlp) and not with yt-dlp, the issue should be raised in the youtube-dl project.
112 ### Are you willing to share account details if needed?
114 The maintainers and potential contributors of the project often do not have an account for the website you are asking support for. So any developer interested in solving your issue may ask you for account details. It is your personal discression whether you are willing to share the account in order for the developer to try and solve your issue. However, if you are unwilling or unable to provide details, they obviously cannot work on the issue and it cannot be solved unless some developer who both has an account and is willing/able to contribute decides to solve it.
116 By sharing an account with anyone, you agree to bear all risks associated with it. The maintainers and yt-dlp can't be held responsible for any misuse of the credentials.
118 While these steps won't necessarily ensure that no misuse of the account takes place, these are still some good practices to follow.
120 - Look for people with `Member
` (maintainers of the project) or `Contributor
` (people who have previously contributed code) tag on their messages.
121 - Change the password before sharing the account to something random (use [this](https://passwordsgenerator.net/) if you don't have a random password generator).
122 - Change the password after receiving the account back.
127 # DEVELOPER INSTRUCTIONS
129 Most users do not need to build yt-dlp and can [download the builds](https://github.com/yt-dlp/yt-dlp/releases) or get them via [the other installation methods](README.md#installation).
131 To run yt-dlp as a developer, you don't need to build anything either. Simply execute
135 To run the test, simply invoke your favorite test runner, or execute a test file directly; any of the following work:
137 python -m unittest discover
138 python test/test_download.py
142 See item 6 of [new extractor tutorial](#adding-support-for-a-new-site) for how to run extractor specific test cases.
144 If you want to create a build of yt-dlp yourself, you can follow the instructions [here](README.md#compile).
147 ## Adding new feature or making overarching changes
149 Before you start writing code for implementing a new feature, open an issue explaining your feature request and atleast one use case. This allows the maintainers to decide whether such a feature is desired for the project in the first place, and will provide an avenue to discuss some implementation details. If you open a pull request for a new feature without discussing with us first, do not be surprised when we ask for large changes to the code, or even reject it outright.
151 The same applies for changes to the documentation, code style, or overarching changes to the architecture
154 ## Adding support for a new site
156 If you want to add support for a new site, first of all **make sure** this site is **not dedicated to [copyright infringement](https://www.github.com/ytdl-org/youtube-dl#can-you-add-support-for-this-anime-video-site-or-site-which-shows-current-movies-for-free)**. yt-dlp does **not support** such sites thus pull requests adding support for them **will be rejected**.
158 After you have ensured this site is distributing its content legally, you can follow this quick list (assuming your service is called `yourextractor
`):
160 1. [Fork this repository](https://github.com/yt-dlp/yt-dlp/fork)
161 1. Check out the source code with:
163 git clone git@github.com:YOUR_GITHUB_USERNAME/yt-dlp.git
165 1. Start a new git branch with
168 git checkout -b yourextractor
170 1. Start with this simple template and save it to `yt_dlp/extractor/yourextractor.py
`:
174 from .common import InfoExtractor
177 class YourExtractorIE(InfoExtractor):
178 _VALID_URL = r'https?://(?:www\.)?yourextractor\.com/watch/(?P<id>[0-9]+)'
180 'url': 'https://yourextractor.com/watch/42',
181 'md5': 'TODO: md5 sum of the first 10241 bytes of the video file (use --test)',
185 'title': 'Video title goes here',
186 'thumbnail': r're:^https?://.*\.jpg$',
187 # TODO more properties, either as:
189 # * MD5 checksum; start the string with md5:
190 # * A regular expression; start the string with re:
191 # * Any Python type (for example int or float)
195 def _real_extract(self, url):
196 video_id = self._match_id(url)
197 webpage = self._download_webpage(url, video_id)
199 # TODO more code goes here, for example ...
200 title = self._html_search_regex(r'<h1>(.+?)</h1>', webpage, 'title')
205 'description': self._og_search_description(webpage),
206 'uploader': self._search_regex(r'<div[^>]+id="uploader"[^>]*>([^<]+)<', webpage, 'uploader', fatal=False),
207 # TODO more properties (see yt_dlp/extractor/common.py)
210 1. Add an import in [`yt_dlp/extractor/extractors.py
`](yt_dlp/extractor/extractors.py).
211 1. Run `python test/test_download.py TestDownload.test_YourExtractor
`. This *should fail* at first, but you can continually re-run it until you're done. If you decide to add more than one test, the tests will then be named `TestDownload.test_YourExtractor
`, `TestDownload.test_YourExtractor_1
`, `TestDownload.test_YourExtractor_2
`, etc. Note that tests with `only_matching
` key in test's dict are not counted in. You can also run all the tests in one go with `TestDownload.test_YourExtractor_all
`
212 1. Make sure you have atleast one test for your extractor. Even if all videos covered by the extractor are expected to be inaccessible for automated testing, tests should still be added with a `skip
` parameter indicating why the particular test is disabled from running.
213 1. Have a look at [`yt_dlp/extractor/common.py
`](yt_dlp/extractor/common.py) for possible helper methods and a [detailed description of what your extractor should and may return](yt_dlp/extractor/common.py#L91-L426). Add tests and code for as many as you want.
214 1. Make sure your code follows [yt-dlp coding conventions](#yt-dlp-coding-conventions) and check the code with [flake8](https://flake8.pycqa.org/en/latest/index.html#quickstart):
216 $ flake8 yt_dlp/extractor/yourextractor.py
218 1. Make sure your code works under all [Python](https://www.python.org/) versions supported by yt-dlp, namely CPython and PyPy for Python 3.6 and above. Backward compatibility is not required for even older versions of Python.
219 1. When the tests pass, [add](https://git-scm.com/docs/git-add) the new files, [commit](https://git-scm.com/docs/git-commit) them and [push](https://git-scm.com/docs/git-push) the result, like this:
221 $ git add yt_dlp/extractor/extractors.py
222 $ git add yt_dlp/extractor/yourextractor.py
223 $ git commit -m '[yourextractor] Add extractor'
224 $ git push origin yourextractor
226 1. Finally, [create a pull request](https://help.github.com/articles/creating-a-pull-request). We'll then review and merge it.
228 In any case, thank you very much for your contributions!
230 **Tip:** To test extractors that require login information, create a file `test/local_parameters.json
` and add `"usenetrc": true
` or your username and password in it:
233 "username": "your user name",
234 "password": "your password"
238 ## yt-dlp coding conventions
240 This section introduces a guide lines for writing idiomatic, robust and future-proof extractor code.
242 Extractors are very fragile by nature since they depend on the layout of the source data provided by 3rd party media hosters out of your control and this layout tends to change. As an extractor implementer your task is not only to write code that will extract media links and metadata correctly but also to minimize dependency on the source's layout and even to make the code foresee potential future changes and be ready for that. This is important because it will allow the extractor not to break on minor layout changes thus keeping old yt-dlp versions working. Even though this breakage issue may be easily fixed by a new version of yt-dlp, this could take some time, during which the the extractor will remain broken.
245 ### Mandatory and optional metafields
247 For extraction to work yt-dlp relies on metadata your extractor extracts and provides to yt-dlp expressed by an [information dictionary](yt_dlp/extractor/common.py#L91-L426) or simply *info dict*. Only the following meta fields in the *info dict* are considered mandatory for a successful extraction process by yt-dlp:
249 - `id
` (media identifier)
250 - `title
` (media title)
251 - `url
` (media download URL) or `formats
`
253 The aforementioned metafields are the critical data that the extraction does not make any sense without and if any of them fail to be extracted then the extractor is considered completely broken. While, in fact, only `id
` is technically mandatory, due to compatibility reasons, yt-dlp also treats `title
` as mandatory. The extractor is allowed to return the info dict without url or formats in some special cases if it allows the user to extract usefull information with `--ignore-no-formats-error
` - Eg: when the video is a live stream that has not started yet.
255 [Any field](yt_dlp/extractor/common.py#219-L426) apart from the aforementioned ones are considered **optional**. That means that extraction should be **tolerant** to situations when sources for these fields can potentially be unavailable (even if they are always available at the moment) and **future-proof** in order not to break the extraction of general purpose mandatory fields.
259 Say you have some source dictionary `meta
` that you've fetched as JSON with HTTP request and it has a key `summary
`:
262 meta = self._download_json(url, video_id)
265 Assume at this point `meta
`'s layout is:
269 "summary": "some fancy summary text",
271 "name": "uploader name"
277 Assume you want to extract `summary
` and put it into the resulting info dict as `description
`. Since `description
` is an optional meta field you should be ready that this key may be missing from the `meta
` dict, so that you should extract it like:
280 description = meta.get('summary') # correct
286 description = meta['summary'] # incorrect
289 The latter will break extraction process with `KeyError
` if `summary
` disappears from `meta
` at some later time but with the former approach extraction will just go ahead with `description
` set to `None
` which is perfectly fine (remember `None
` is equivalent to the absence of data).
292 If the data is nested, do not use `.get
` chains, but instead make use of the utility functions `try_get
` or `traverse_obj
`
294 Considering the above `meta
` again, assume you want to extract `["user"]["name"]
` and put it in the resulting info dict as `uploader
`
297 uploader = try_get(meta, lambda x: x['user']['name']) # correct
301 uploader = traverse_obj(meta, ('user', 'name')) # correct
307 uploader = meta['user']['name'] # incorrect
311 uploader = meta.get('user', {}).get('name') # incorrect
315 Similarly, you should pass `fatal=False
` when extracting optional data from a webpage with `_search_regex
`, `_html_search_regex
` or similar methods, for instance:
318 description = self._search_regex(
319 r'<span[^>]+id="title"[^>]*>([^<]+)<',
320 webpage, 'description', fatal=False)
323 With `fatal
` set to `False
` if `_search_regex
` fails to extract `description
` it will emit a warning and continue extraction.
325 You can also pass `default=<some fallback value>
`, for example:
328 description = self._search_regex(
329 r'<span[^>]+id="title"[^>]*>([^<]+)<',
330 webpage, 'description', default=None)
333 On failure this code will silently continue the extraction with `description
` set to `None
`. That is useful for metafields that may or may not be present.
336 Another thing to remember is not to try to iterate over `None
`
338 Say you extracted a list of thumbnails into `thumbnail_data
` using `try_get
` and now want to iterate over them
341 thumbnail_data = try_get(...)
344 } for item in thumbnail_data or []] # correct
350 thumbnail_data = try_get(...)
353 } for item in thumbnail_data] # incorrect
356 In the later case, `thumbnail_data
` will be `None
` if the field was not found and this will cause the loop `for item in thumbnail_data
` to raise a fatal error. Using `for item in thumbnail_data or []
` avoids this error and results in setting an empty list in `thumbnails
` instead.
359 ### Provide fallbacks
361 When extracting metadata try to do so from multiple sources. For example if `title
` is present in several places, try extracting from at least some of them. This makes it more future-proof in case some of the sources become unavailable.
366 Say `meta
` from the previous example has a `title
` and you are about to extract it. Since `title
` is a mandatory meta field you should end up with something like:
369 title = meta['title']
372 If `title
` disappears from `meta
` in future due to some changes on the hoster's side the extraction would fail since `title
` is mandatory. That's expected.
374 Assume that you have some another source you can extract `title
` from, for example `og:title
` HTML meta of a `webpage
`. In this case you can provide a fallback scenario:
377 title = meta.get('title') or self._og_search_title(webpage)
380 This code will try to extract from `meta
` first and if it fails it will try extracting `og:title
` from a `webpage
`.
383 ### Regular expressions
385 #### Don't capture groups you don't use
387 Capturing group must be an indication that it's used somewhere in the code. Any group that is not used must be non capturing.
391 Don't capture id attribute name here since you can't use it for anything anyway.
396 r'(?:id|ID)=(?P<id>\d+)'
401 r'(id|ID)=(?P<id>\d+)'
404 #### Make regular expressions relaxed and flexible
406 When using regular expressions try to write them fuzzy, relaxed and flexible, skipping insignificant parts that are more likely to change, allowing both single and double quotes for quoted values and so on.
410 Say you need to extract `title
` from the following HTML code:
413 <span style="position: absolute; left: 910px; width: 90px; float: right; z-index: 9999;" class="title">some fancy title</span>
416 The code for that task should look similar to:
419 title = self._search_regex( # correct
420 r'<span[^>]+class="title"[^>]*>([^<]+)', webpage, 'title')
426 title = self._search_regex( # correct
427 r'<span[^>]+class=(["\'])title\1[^>]*>(?P<title>[^<]+)',
428 webpage, 'title', group='title')
431 Note how you tolerate potential changes in the `style
` attribute's value or switch from using double quotes to single for `class
` attribute:
433 The code definitely should not look like:
436 title = self._search_regex( # incorrect
437 r'<span style="position: absolute; left: 910px; width: 90px; float: right; z-index: 9999;" class="title">(.*?)</span>',
438 webpage, 'title', group='title')
444 title = self._search_regex( # incorrect
445 r'<span style=".*?" class="title">(.*?)</span>',
446 webpage, 'title', group='title')
449 Here the presence or absence of other attributes including `style
` is irrelevent for the data we need, and so the regex must not depend on it
452 ### Long lines policy
454 There is a soft limit to keep lines of code under 100 characters long. This means it should be respected if possible and if it does not make readability and code maintenance worse. Sometimes, it may be reasonable to go upto 120 characters and sometimes even 80 can be unreadable. Keep in mind that this is not a hard limit and is just one of many tools to make the code more readable
456 For example, you should **never** split long string literals like URLs or some other often copied entities over multiple lines to fit this limit:
461 'https://www.youtube.com/watch?v=FqZTN594JQw&list=PLMYEtVRpaqY00V9W81Cwmzp6N6vZqfUKD4'
467 'https://www.youtube.com/watch?v=FqZTN594JQw&list='
468 'PLMYEtVRpaqY00V9W81Cwmzp6N6vZqfUKD4'
473 Extracting variables is acceptable for reducing code duplication and improving readability of complex expressions. However, you should avoid extracting variables used only once and moving them to opposite parts of the extractor file, which makes reading the linear flow difficult.
480 title = self._html_search_regex(r'<title>([^<]+)</title>', webpage, 'title')
486 TITLE_RE = r'<title>([^<]+)</title>'
487 # ...some lines of code...
488 title = self._html_search_regex(TITLE_RE, webpage, 'title')
492 ### Collapse fallbacks
494 Multiple fallback values can quickly become unwieldy. Collapse multiple fallback values into a single expression via a list of patterns.
501 description = self._html_search_meta(
502 ['og:description', 'description', 'twitter:description'],
503 webpage, 'description', default=None)
510 self._og_search_description(webpage, default=None)
511 or self._html_search_meta('description', webpage, default=None)
512 or self._html_search_meta('twitter:description', webpage, default=None))
515 Methods supporting list of patterns are: `_search_regex
`, `_html_search_regex
`, `_og_search_property
`, `_html_search_meta
`.
518 ### Trailing parentheses
520 Always move trailing parentheses after the last argument.
522 Note that this *does not* apply to braces `}
` or square brackets `]
` both of which should closed be in a new line
529 lambda x: x['ResultSet']['Result'][0]['VideoUrlSet']['VideoUrl'],
536 lambda x: x['ResultSet']['Result'][0]['VideoUrlSet']['VideoUrl'],
542 ### Use convenience conversion and parsing functions
544 Wrap all extracted numeric data into safe functions from [`yt_dlp/utils.py
`](yt_dlp/utils.py): `int_or_none
`, `float_or_none
`. Use them for string to number conversions as well.
546 Use `url_or_none
` for safe URL processing.
548 Use `try_get
`, `dict_get
` and `traverse_obj
` for safe metadata extraction from parsed JSON.
550 Use `unified_strdate
` for uniform `upload_date
` or any `YYYYMMDD
` meta field extraction, `unified_timestamp
` for uniform `timestamp
` extraction, `parse_filesize
` for `filesize
` extraction, `parse_count
` for count meta fields extraction, `parse_resolution
`, `parse_duration
` for `duration
` extraction, `parse_age_limit
` for `age_limit
` extraction.
552 Explore [`yt_dlp/utils.py
`](yt_dlp/utils.py) for more useful convenience functions.
556 ##### Safely extract optional description from parsed JSON
558 description = traverse_obj(response, ('result', 'video', 'summary'), expected_type=str)
561 ##### Safely extract more optional metadata
563 video = traverse_obj(response, ('result', 'video', 0), default={}, expected_type=dict)
564 description = video.get('summary')
565 duration = float_or_none(video.get('durationMs'), scale=1000)
566 view_count = int_or_none(video.get('views'))
573 See [README.md#embedding-yt-dlp](README.md#embedding-yt-dlp) for instructions on how to embed yt-dlp in another Python program