download_file#, cache=False, show_progress=True, timeout=None, sources=None, pkgname='astropy', http_headers=None, ssl_context=None, allow_insecure=False)[source]#

Downloads a URL and optionally caches the result.

It returns the filename of a file containing the URL’s contents. If cache=True and the file is present in the cache, just returns the filename; if the file had to be downloaded, add it to the cache. If cache="update" always download and add it to the cache.

The cache is effectively a dictionary mapping URLs to files; by default the file contains the contents of the URL that is its key, but in practice these can be obtained from a mirror (using sources) or imported from the local filesystem (using import_file_to_cache or import_download_cache). Regardless, each file is regarded as representing the contents of a particular URL, and this URL should be used to look them up or otherwise manipulate them.

The files in the cache directory are named according to a cryptographic hash of their URLs (currently MD5, so hackers can cause collisions). The modification times on these files normally indicate when they were last downloaded from the Internet.


The URL of the file to download

cachebool or “update”, optional

Whether to cache the contents of remote URLs. If “update”, always download the remote URL in case there is a new version and store the result in the cache.

show_progressbool, optional

Whether to display a progress bar during the download (default is True). Regardless of this setting, the progress bar is only displayed when outputting to a terminal.

timeoutfloat, optional

Timeout for remote requests in seconds (default is the configurable

sourceslist of str, optional

If provided, a list of URLs to try to obtain the file from. The result will be stored under the original URL. The original URL will not be tried unless it is in this list; this is to prevent long waits for a primary server that is known to be inaccessible at the moment. If an empty list is passed, then download_file will not attempt to connect to the Internet, that is, if the file is not in the cache a KeyError will be raised.

pkgnamestr, optional

The package name to use to locate the download cache. i.e. for pkgname='astropy' the default cache location is ~/.astropy/cache.

http_headersdict or None

HTTP request headers to pass into urlopen if needed. (These headers are ignored if the protocol for the name_or_obj/sources entry is not a remote HTTP URL.) In the default case (None), the headers are User-Agent: some_value and Accept: */*, where some_value is set by

ssl_contextdict, optional

Keyword arguments to pass to ssl.create_default_context when downloading from HTTPS or TLS+FTP sources. This can be used provide alternative paths to root CA certificates. Additionally, if the key 'certfile' and optionally 'keyfile' and 'password' are included, they are passed to ssl.SSLContext.load_cert_chain. This can be used for performing SSL/TLS client certificate authentication for servers that require it.

allow_insecurebool, optional

Allow downloading files over a TLS/SSL connection even when the server certificate verification failed. When set to True the potentially insecure download is allowed to proceed, but an AstropyWarning is issued. If you are frequently getting certificate verification warnings, consider installing or upgrading certifi package, which provides frequently updated certificates for common root CAs (i.e., a set similar to those used by web browsers). If installed, Astropy will use it automatically.


Returns the local path that the file was download to.


Whenever there’s a problem getting the remote file.


When a file was requested from the cache but is missing and no sources were provided to obtain it from the Internet.


Because this function returns a filename, another process could run clear_download_cache before you actually open the file, leaving you with a filename that no longer points to a usable file.