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When you are logging in, your Synapse credentials can be stored using the rememberMe parameter. You should use the rememberMe feature only if you are on a personal computer where you are the only Synapse user. Visit the Python client docs and R client docs for examples.
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Note the period at the beginning of the file name that makes it a hidden system file on Linux-like operating systems, since it will contain sensitive information.
When you install the Synapse Python client, a .synapseConfig file filled with examples should appear in your home directory if a configuration file is not already present. You can uncomment and fill out the parameters of interest. A copy of the default .synapseConfig template can also be obtained here.
See the following for more information on managing credentials in the .synapseConfig